In a lot of discussions about free spirits, ghouls, quickening and others there is often the argument, that wards are all over the place.

I tried to guesstimate, how many wards can be built, given that the number of mages is limited.

Let's take Seattle with 3 million people as an example.
That means there are 30000 magically talented people.
To build a good ward with rating 4 which offers some resistance you would need somebody with Magic 4 (others could do it, too, but would risk physical damage)
We don't have demographics of Magic Ratings, but I guess that only a fifth of the magically talented people would have Magic 4 or more, YMMV.
So that would be 6000 mages with Magic 4+; potential ward 4 builders.

To build a rating 4 ward takes 4 hours, so one could build two per day as a full time job. Building wards has some risk that somebody might track the ward to its creator to knock him out and a lot of mages will have better things to do than building wards.

I decided for my table, that only 1 in 10 mages would do this like a full-time job (in reality even less will do it full-time, but if I add the half-time builders and the sometimes builder, I count it on the average as 1 in 10).
That would be like having 600 mages building wards with rating 4 as a full-time job, two wards per day.
600 mages can keep 12000 wards running (Magic 4 + Willlpower 3 = 2.33 weeks ward, let's say 2 weeks until the mage has to come and rebuild the ward, with 5 days work a week that would be 5 * 2 (per day) * 2 (weeks duration) 20 wards per mage)
A German statistic says that there are about 5 people per building on the average (don't know the corresponding American number) so there would be 600000 buildings in Seattle.
We have 12000 wards for 3 million people and for 600000 buildings. You would find 1 ward per 250 people and 1 ward per 50 buildings.

So imagine standing on a middle class street. Looking up and down the street, left and right for 12 buildings there would on the average be 1 ward.

I think that might be a very similar number to the number of safes and it can be done with an average of one in ten mages building wards full-time (on the average).
You might add a number of mages who don't want to risk repercussions and summon spirits to let them build a ward. Since there is no feedback when the ward is attacked, this would be a ward without functioning alarm, but it would at least be a wall in astral space.

I was always doubtful, that there would be enough mages to build so many wards that they become a common thing, but it seems it can be done.
This is a calculation to see if the order of magnitude would fit. YMMV.

Are all ~8% of these buildings in your figuring going to be wholly within the 200 cubic meter limit of these wards? (Those that can create warded areas are limited to Magic times fifty cubic meters per ward.) Assuming a height of 3m from floor to ceiling, this leaves a floor coverage of 66 2/3 square meters (or roughly 717 and a half square feet for us old American folks). As an example, this is slightly less than half of my home (roughly 150 square meters), doesn't include a garage. I can't speak for most people, but I'd go all or nothing in coverage.

I'm also assuming that these same ~8% are going to be able to afford the 400 (general charge is 100 per hour; SR4a, page 194) it's going to cost every time a mage comes to visit (meaning roughly 800 per month, or 10,400 annually).

It's only 2% of buildings (1 in 50), but I didn't express it well, when I tried to illustrate this with a a character standing on the street looking down the left side 12 buildings, right side 12 and looking up the street left and right there each with 12 buildings one of them would have the ward.

I do see the argument, that a safe and good locks are one-time investments and that a ward is rather like a security service which comes by on patrol and has monthly fees.
I couldn't find how much it will cost, when a security firm comes visiting say once a night (my google-was weak) but perhaps somebody can help me with that.

It does cost a lot with about 800 nuyen per month. You won't find this in Puyallup and if you do you'll know there is probably a hot criminal activity, that makes a lot of money.

Of course it would be nicer, if I could afford to cover my complete house with a ward, but with this pricing it will be only one floor of my house or an apartment of about 66 square meters.

You could even have interior designers modeling apartments since the 2030s that are "ward friendly" by putting "interesting" rooms in a square where a ward can be build and leaving other rooms where one is not so often outside the "ward square".

Another thing is that wards build by summoned spirits combined with some GloMoss to see if the ward is still there will be cheaper, because it has no alarm function and the mage has to do nearly nothing to let it be built (he has a very low risk of taking 8 Stun). It would be like having a wall that keeps out random spirits and mages who will probably search for easier targets.
Additionally without tracing the ward you cannot see, if it is a complete ward built by a mage or a ward built by spirit who won't notice or won't care, if it is attacked.

The other thing to factor into that equation is: How many people/businesses are going to really need astral security on that scale.
also, how much is having a watcher spirit do patrols of a neighborhood and reporting suspicious astral activity to a mage who then informs the relevant householder? cos i can see that being a very cost-effective budget solution as well.

One of the big problem with your numbers is that your using the global average for the number of awekened, when the fact is that a city like Seattle would have much higher number of awekened, especially the kind of that can do needed jobs.

Aren't all Seattle's awakened population shadowrunners? i know that about half of them seem to have ended up in my group

In the long run it might be cheaper to just have pots of glowmoss and other such astrally sensitive plants around your area. If you don't have a lot of astral activity, no need to invest in the extra security. Good biofiber looks like it's more expensive in the short run, but as a passive deterrent it could be cheaper in the long run (I've not run the numbers in cost and area).

While it can be good to try and build a statistical macromodel for your table, you are always gonna be using a lot of assumptions which may or may not be right. Your assumptions do not seem too unreasonable, but given that SR canon suggests that wards are not uncommon in the 6th world, your conclusions make it pretty clear that the devs did not really think about things when they wrote the rules for creating wards.

Off the top of my head, 1 in 5 mages with Magic 4+ might be a little low given that most corps would want to train their wage mages up to be able to make wards. Also, spirits can be used for teamwork to reduce the time it takes to create a ward, for instance. On the flip side, you starting pool of awakened characters does not take Adepts and Spell Knack folk, who are magically active but cannot create wards, into consideration.

If you were to lo-ball wards in your campaign, it will have the knock-on effect of making things a little too easy for awakened characters (never having to recast sustained/focus sustained spells, can always take spirits with them, astral recon a no-brainer breeze etc etc). So I think you probably need to take game balance into consideration when you make your decision.

The way I play it, most corp facilities and major buildings will have a few warded areas (usually highly sensitive R&D areas, plus perhaps top executive floors etc). High end clubs and the like may well have wards across their entrances, as will security-conscious public buildings such as airports etc. Large areas can be warded more efficiently by using separate 1m thick wards for each wall, floor (if necessary) and ceiling. This helps limit mages doing things like (1) casting spells into a sustaining focus using Edge to get bazillions of hits (spells have to be recast after going through each ward unless they force their way through and raise the alarm), and (2) using Invisibility and/or Concealment to waltz unchallenged with tons of hardware into a high-security area every time.

I don't think Spell Knacks are an option (unless a GM allows them to also take Astral Sight). You have to be able to astrally perceive in order to make a ward. Means Mystics and Adepts (yes, Adepts; they're specifically mentioned as being able to on SR4, page 194) must have the Astral Perception power, and might include those with the Astral Sight quality (though I wouldn't count on the latter with a capped Magic of 1).

There's also the option of using lower force wards just for the warning when they're broken through/destroyed.

I do have several assumptions in this guesstimation, but to my surprise it comes up with fitting numbers. I'm pretty sure the devs never thought about this and so it's surprising that something follows that is not completely silly.

I think that it at least shows that you would have a hard time to explain how for example every fifth building could have a ward. This would mean that every mage with Magic 4+ would have to work full-time building wards or that you have a lot more Magic 4+ mages than I assumed or both.
It's a nice upper limit that fits the idea that not everyone can be warded but the rich can afford it and there are enough mages to do it.

About the neccessity of this kind of astral security:
If it were cheaper, I guess everybody would like to be protected from spying (by clairvoyance or a materialized spirit next room who tells his summoner via mental link every word he hears) and from being killed or manipulated by ritual magic.

I think you cannot protect yourself from being killed at home by a spirit that materializes as soon as you step outside your ward, but with a ward in those areas where you are for several hours (bedroom, living room, office) you are somewhat protected against rituals, because I would rule that if you are inside a ward even for a part of the time a ritual takes to be cast, the ward penalty must be taken in account.
Same for the spying spirit. I will at least know when the spirit tries to break in to spy on me.

By the way: Biofiber needs weekly refilling. It is very, very expensive if you use it for an apartment or even more. You have to use is for a small room to be as cheap as a ward. I think Biofiber is for people who don't like the idea of having a mage or anybody come by and see the place that is protected. We don't want strangers here. Biofiber refillings can be done by the owner of the place without involving strangers and will be used where this kind of privacy is valued.
It costs 5 nuyen per sq meter per week per Force and that makes it a little difficult to compare with a ward.
For a rating 4 ward that would be 200 cubic meters which are 80 sq meters, if the ceiling height is 2.5 m
If I have 80 sq meters formed like 8 m times 10 m with 2.5 m height I have walls and ceilings of 160 + 36 = 196 sqm. That would be about 196 * 4 week * force 4 * 5 nuyen per month. That's about 15000 nuyen per month.

It's not a big problem, that I think that only about 1 in 50 buildings will have a ward and only 1 in 250 people (including kids) will have one at home or at work. It sounds low, but of course shadowrunners will encounter them a lot more often, because they break in all the time in protected and/or rich places.
It's just like big safes. John Q Normal will not see one very often, burglars have to deal with them all the time.
Game balance is still ok

I was just trying to see how many shops, houses, apartments will be a problem for those poor free spirits and ghouls. They at least have the advantage, that they can see, that this or that shop has a warding and will try to work around.
The mage with the quickened spell will not see it, when he steps in to the shop and his quickened spell fights the ward. Granted, a rating 4 ward is not that big a danger depending on the force of the quickened spell, but rolling dice will get you some time.
With only about 1 in 50 shops the danger is relatively low and if I had a quickened spell I would probably go through the city with astral perception to see where the shops are that I shouldn't visit. With 1 in 50 the danger of suddenly having a new ward in one of the shops I regularly visit is relatively low.

Edit: forgot a "not" in the sentence with John Q Normal.

He's using the global average, witch i'm pretty sure now days not only includes those, but also every other kind of awakened.

If where talking about buildings that a runners team is breaking in or really even going to on business, not in the lightest.

And here's your biggest mistake.
Even if only 1 in 50 buildings in Seattle has a ward, that definely doesn't mean that only 1 in 50 shops is warded.

Let's take Seattle with 3 million people as an example.
That means there are 30000 magically talented people.
To build a good ward with rating 4 which offers some resistance you would need somebody with Magic 4 (others could do it, too, but would risk physical damage)
We don't have demographics of Magic Ratings, but I guess that only a fifth of the magically talented people would have Magic 4 or more, YMMV.
So that would be 6000 mages with Magic 4+; potential ward 4 builders.

more than 1/2 are Adepts and some Mages coldn't cope with awakening so I'd estimate that instead of 6000
You'd have ca 2500 Mages that are willing & able to erect a Ward lvl 4

with a Willingness to Dance
Medicineman

There's also another thing to consider with wards, and anything that is dual natured: Attention. If the wild spirits just hang around in astral space it means they're relatively ignoring what's going on in the physical and not interacting with it. Wards exist in the astral, typically being quite visible compared to the dull auras of objects and most sapients & critters (unless they're masking or polarized wards, which are a lot harder to make). Think of moths to a flame; if you don't want tons of moths around your home then you don't have a lot of lights around your home. If there's a lot of astral activity, you're bound to attract spirits and the like, just like you're bound to attract dogs and bears if you leave out rotting garbage.

It's a good sign, when some say one fifth of the awakened have Magic 4+ is too high, and some say it's too low.

As said, I only wanted to see if the order of magnitude is ok and to me it is. Of course YMMV.

Is the wild spirit problem so big, that one should rather not have wards?
I personally doubt it, but it might be a nice addition for the descriptions of some wards that shadowrunners encounter. They will then sometimes see a wild spirit hanging around there.
Also, I would rather have security by having a "light" (ward) and some false alarms by wild spirits that attacked the ward just for the fun of it, than not having any security (always with the assumption that I have enough money to afford a ward).

You might also add to your assumptions that the personal homes of mages who can raise a ward will be warded, which will add a couple more to the average.