Yeah, Wraith's estimate sounds about right.
Here's my reasoning. For a major sprawl like Seattle, the AAA's will have on average 3 "go-to-local" teams each (some AAA's only one or two, some 4 or 5, and then there could be some cross-over teams). So that's about 30 teams x 4.5 members = 135. Then each of the Big Syndicates will have 1 or 2, closer to 1 as they like to use "in-house" more often than not. So there's another ~6 teams x 4.5 = 27. 135 + 27 = 162.
Now "realistically" this sounds kind of high to me, but having 3 teams for each of the AAA's means I can have 1 fully fleshed out as antagonists (or contacts), 1 that's "named" and mentioned but not fleshed out, and 1 that I don't bother talking about for when a player says something like, "Hey, I just read that one Sourcebook/Novel/etc., and there was this one team with this one guy...I want to go find him," or "hey, would that corp have a group that specialized in X [and yeah, it making sense to me, but I didn't think of it until the player now mentions it], shouldn't we try to find those guys?"
Even then though, it could easily be consolidated to 2 teams per AAA.
If runners pull a run, on average, every 2 weeks, and there's 36 teams (of high BP build), that puts it at 72 runs a month, or about 18 runs in a week.
If runners pull a run, on average, every month, and there's 36 teams (of high BP build), that puts it at 36 a month, or about 9 runs in a week.
I like the lower end number of runs as 18 a week for they pay that high BP builds would expect means it's an expensive and busy city filled with more mayhem that the powers that be would like/allow. Plus, I like writing up little blurbs of more noticeable (or failed) runs by "those other runners in the sprawl" so the more actual runs means the more I'm likely to write up which I just don't have a lot of time to do these days

But for actual game play, I agree with The Cat. "What should be happening for this run" and rarely is another group of shadowrunners called in for security purposes...they tend to be too expensive.
