We could also talk generally about brain-eating strategy. I mean, at the end of the day there will be one lynch. Should all zombies try to eat that brain? Should zombies flip a coin and go 50/50 between eating that brain and trying to predict the scum kill and targeting that brain for eating? If we all go for the town lynch brain, than any potential scum zombies would get to eat the nightkill victim's brain unopposed. And that means even fewer brains for town subsequent nights.
Honestly there can't be too many zombies at least to start, because the math is not great. Every zombie has to eat a brain every other night, each zombie can only eat one brain, and there's only going to be 2 new brains generated every day/night cycle (most likely). So mathematically, there's a maximum number of zombies the game can support without some number being guaranteed to Romero.
How can we work this out? If every zombie has to eat every other night, than each zombie needs 1/2 brain per day/night cycle. The game is making 2 brains per day/night cycle. Therefore, a sustainable number of zombies would be 4. That means that <=4 zombies is uninteresting -- they can all get along. Although, zombies can also eat brains they don't need to deprive other zombies of brains, so 4 or less isn't a guarantee of everyone eating well.
Also, zombies can only eat human brains! So the expected number of brains per night also needs to be modified by the chance that the kill was not a zombie. And we don't know that unless we know the thing we're trying to figure out, the number of zombies. The more zombies there are, the fewer brains there are to go around.
Considering all of this, and assuming there aren't mechanics that turn humans into zombies, I'm betting we have 4 or 5 zombies. Any more would make the competition for brains too fierce; less would make it too easy.