Assuming you're using the default zombie bots, you can make a round system that incrementally adds more and more zombies as time / rounds go on.
To start you'd want to place many zombie spawns. If you'd like to keep zombies spawning throughout the round, simply name the zombie botholes R1, R2, R3, etc.. (For the rounds). Event all that will not be active on the first round to have no bot (onbotspawn > self > Setbottype > none), and you're good for that. If you only want one bot to spawn from each bothole, and then when killed, no more to spawn from that point until the round is over, you must add two events to all those holes being:
The previous event above (onbotspawn > self > Setbottype > none). Again, only apply this to spawns that will not be active the first round. And:
Onbotdeath(or killed, don't remember which one it is) > Self > Setbottype > None.
After you've placed all the spawn points, you need to make a clock. This will act as your timer for when a new round will start. (If you wanted new rounds to start only when all zombies are killed, sorry, but that's not possible with events).
This clock can be set up multiple ways, either using relays, projectiles, or some other event that might be specifically made for making timers. I like projectile clocks, which are completely default and fairly easy to make. You simply need a start round brick, that when clicked, will fire a projectile at another brick, that shoots a projectile at another brick, etc etc. You want to add delays to all of these. At some point you'll make one brick that acts as the next round brick, which, depending on how you made the zombie spawns work previously, you will event.
If you did the constant spawning method, you simply need to add:
Onprojectilehit > namedbrick(R2, R3, etc, depending on what round it is) > Setbottype > Zombie.
If you did the one life system for the zombies, you must add two events, those being:
Onprojectilehit > namedbrick(R1) > Setbottype > zombie (This will respawn all the zombies from first round. You do the same thing for R2, R3, etc, as the rounds go on.
Along with all that, you need to add another timer event to that same brick which will spawn a projectile that hits another brick that continues the timer.
At this point, you can see it'll get fairly repetitive, you just keep adding new zombie spawns and time for the timer. If you need any more help or an example in game just ask and I'll try to get back to you soon. I'm on phone rn so I can't do much more for ya. Hope this helped.
