A friend told me a while back that a maximum of 50 Zombies per/player could appear on the board...but that meant 50 Zombies physically in the map or approaching barriers. If this is true, it means the AI is still sending more Zombies out in the higher levels as the 200 on the board are being blasted and killed.
What I can say for sure is that from the view of the Catwalk, I have seen what appeared to be an endless stream of zombies coming from the Catwalk Window, the Tunnel, the Courtyard and the Mezzanines, to the point where an upgraded Waffle and 2 MG 42's could hardly slow the tide. I don't think it would be an exaggeration to state well over 100 Zombies were in view.
Exciting news! Is this true? Can anyone confirm or debunk this? Please remember, I'm looking for the number of Zombies per/player, per round.