Limiting the number of prefabs?

So I have waves of 50 guys, but I only want 20  or so to be on the screen at once, and once there's the 20 guys I want the oldest to ones to start despawning. I thought that the recycle  oldest checkbox would achieve this but I can't seem to get it to work, once the 20 guys are spawned they just stay there, and once I kill them no one else spawns. I also can't find anything in the documentation about recycle oldest.
Thanks for your time