easiest way to up your frames per sec is to lower the graphics draw on your machine
use the full shaders, but quick light mapping. it looks almost as good and it runs like ten times faster
and try decreasing the game res and/or the texture res
if you can do that then yo won't need to worry about the number of AI in your game at any given point
now of course, having a street with hundreds upon hundreds of zombies to your left and right (each with one of 5 different AI) would probably give you a little bit of a problem
just a little bit
that's all i had anyways
as you can see my game is going to need to be scaled down quite a bit
"There's more to life than being really, really, ridiculously good looking." - Derek Zoolander