If you have a super tanky top of the line Server PC, you could probably swing as many as 64 by disabling horde nights. Maybe. But I wouldn't recommend it, as it would likely have a lot of issues.
And to note, The maps are at most 10km squared, and even 16 can start feeling very crowded very quickly.
And also to note, every additional player is an additional number of zombies active at one time, but the server has a built in limit (that is adjustable) of 80 by default; So with a default horde night setting of 8 zombies per person, no more than 10 people would have zombies- This also is a problem just when a lot of people are wandering around, if one player has 20 zombies loaded around them, maybe some guy across the map has less than he should or even none. So definitely tune this value to your use case. (I'd suggest a... Read more