tbh it seems to vary a little bit. Sometimes I run into "hordes" that only seem to be 4-5 zombies big. Other times it can be as many as 10 or 11. And sometimes they throw in a "freak" zombie type, like a Big Bastard, Feral Zombie, SWAT Zombie, etc.
I hate freak zombies. I only have met one but man was he hard to kill.