I don't think bouncers spawn automatically; players need to buy them from the bot spawners. Depending on how much cash the enemy team has, you could be facing quite a few bouncers at any time.
Bouncers don't spawn every wave on Spunky Cola. *To MrRedDragon: Spunky Cola is the only arena where Pro spawned bots automatically spawn, which in this case are Bouncers and I want to say every other wave once the match has gone on for a set amount of time. Haven't paid much attention to how frequent and at what wave/time they start spawning at. The only reason I, and I will assume many others, dislike the bouncers spawning is due to the drop in framerate. Other then that I like it.