Server mod that, if a bot dies, they can't respawn until only x number of bots are alive.
That wasn't probably the best way to explain it...
Say the limit was set to 8. Basically, if there are 8 or more bots still alive, they won't respawn. If one dies, one will randomly spawn from the ones that are dead.
If a minigame starts/resets, don't worry about how many bots there are, just when some of them die.
Also the limit is changeable.