So, this might be a total newb question...
One of my players wants to know how many spirits he can summon/keep control of at one time. Near as I can tell, SR3 doesn't list any sort of cap based on Magic or Cha, nor does it impose a penalty for summoning more spirits based on the number you've summoned already.

What's stopping him, prior to a fight, from summoning spirits every round for 10 minutes (100 spirits), put them on standby, and then spend every round of a combat telling two spirits to attack the baddies?

Is this another example of GM-discretion-is-needed, even though it probably should be covered in the main rules?

If it is, what is a sensible solution? Should max spirits == Magic x 2. Perhaps make the drain test harder by adding (Total Force of All Spirits on Standby) / 2 to the drain target number or somesuch.

Charisma is the limit to the number of spirits a character may have bound at one time. The character can bind an additional number of Watchers equal to Charisma. Shamans can only have one spirit under "immediate" control limited by Domain, though they can have up to their Charisma in spirits of different Domains performing their last service. Ally spirits are not counted towards these limits, but they have a built in limit (as they require a point of the character's Magic upon summoning). This is covered in the main rule books, SR3 & MitS, as I recall, though the page number escapes me.

Sweet. Thanks a lot. Was that in Magic in the Shadows (which I don't have in my office at the moment), or am I an idiot for not finding it in SR3?

I edited it, probably while you read it.

Well:

The nature spirits limit is in SR3, P184, The 4th and 5th paragraphs under the Domains heading. The limit on great-form nature spirits is the summoner's Charisma as described in MitS (P107).

The watchers limit is on P100 of MitS.

The limit on the number of Elementals is in SR3, P186, 1st paragraph under Binding Elementals.