onEnterZone, if the variable is 0 spawn, increment variable. On leave zone, decrement variable, if it's 0 then despawn.
might even work with print counts if you don't have more than nine players, leave it at 9 for 0 and count up from there, when you overflow twice without an underflow, make it disable its normal underflow despawn and relay to another print brick and make it count up one. It should only toggle the events once. If it underflows, have it fire a relay back and increment itself back to 0, when your main brick gets a relay make it reset everything and despawn the bots. If your secondary brick overflows, you will ascend to godhood because you have 100 players on your server it must be new years or something.
I spent way too much time working out the logic of print counts for you there. Use VCE because I probably goofed, but even if I didn't it's just simpler to use in general and it's useful