There are a few ways to do this.. I'd recommend checking out the ALiVE wiki:

http://alivemod.com/wiki/index.php/Virtual_AI_System
"Active Limiter" in VAI module

You can also set the AI spawn numbers in the various placement modules:
http://alivemod.com/wiki/index.php/Military_Placement_(Military_Objectives )

You can also tweak the numbers in the CQB modules:
http://alivemod.com/wiki/index.php/CQB