[IceCube!]

There often arent a lot of zombies as people host them in hosts, this is expensive as each Zombie NPC costs you one player slot. However I find it's cheaper and better to make zombie serves and host them off a server where you have control of the server and how many slots are allocated to that server, this means you can have say 30 players and 470 zombies. Or if you can get the 1000 player server, sole bigger numbers, you just need to find a good number which works and doesn't lag your users.