How do you treat Combat Load Vest in your game? ,my GM gives it 12 slots.
Butt Pack,Ammonition Pouch,Ration Pouch and Smart Canteen takes 1
Slot/each. Free Slot's for 1 Pistol Holster and 1 Knife Sheath.

We just handwave this kind of thing. There are no weights in SR, so we just don't get silly. And nobody in SR ever runs out of ammo, so it doesn't matter how many mags you have. Really, I'm not kidding. It's less common for a PC in SR to change mags than is for Arnold in Commando.

I've used two different types of Load vests in real life:
Webbing:
* Butt pack (can carry rain gear, a couple of rations, and small junk (cam paint, chap stick, etc))
* Two ammo pouches that can carry 4 spare clips of C7 (M16) rounds
* A can-tine pouch
* C9 (LMG) pouch (Often used to carry a first aid kit)
* Bayonet carrier
* KFS carrier
Tactical Vest:
* C9 (LMG pouch (often used to carry rain gear, and small junk)
* Can-tine pouch
* Two First Aid pouches (water sealed)
* Two ammo pouches that can carry 4 spare clips of C7 (M16) rounds
* Bayonet Carrier
* Flash light carrier

Along with the Tac Vest I also had a small pack that easily attached that would give me enough room to carry almost everything you find in a ruck sac.

I would rule that the Combat Load Vest would be similar to the Tac Vest for storage capabilities.

You got a pretty relaxed GM if we fail to to keep accurate ammo count or change mags that's a serious no no for us.
Plus it helps keep things more realistic and challenging instead of being arnie and shooting 100 + rounds from a machine pistol. As far as the weight it is hand waved but if you abuse it our GM will bring in some modifiers for sure so we just keep it at a realistic amount.

OTOH, I am meticulous with my ammo usage in SR, even if the rest of the group (GM included) generally handwave it, unless you're pissing bullets (suppressive fire--which the GM uses a lot).

I changed clips once in some 30 sessions (that's 63 bullets per clip).

No weight? no ammo tracking? gods you have a wussy GM

As a GM (to give you some perspective), I expect my players to track it. I have too much on plate as it is, and this is on the honor system. That being said, in a year of gaming (real time), we've had only....not counting hijacked drones....less than a 100 rounds expended by the entire party. Partly due to the GM style (gun play is usually a sign of failure), and the fact that most time in one combat round the opposition has been defeated.

Pretty much. Nobody ever misses. Targets may dodge (though that is rare also), but how often does someone actually get no successes shooting?

There isn't any reason in SR to not buy whatever you consider to be the absolute best ammo and only shoot it. It really doesn't matter how much it costs, you just won't use that much in most circumstances. Ok, this is true for people who actually used aimed fire, suppression with APDS can get expensive.

If you like suppression fire you'll burn though noticeable amounts of ammo, and there are some other situations I've seen come up where you burn huge amounts of ammo, but for us they have always been with actual machine guns on drones with big belts of ammo providing fire support against major nasties.

But in general, your medium+ lifestyle will cover using half a magazine of ammo once every other week.