Dude let me explain you:
There are maximum of 16 players in a server... let assume that the first zombie got 5 kills instantly and became human , now there are 5 zombies and 11 humans, now again assume that 1 more zombie got 5 kills ( he also get change to human) , now there are 10 zombies and 6 humans and so on ,
Now u understand what I'm trying to say? There can't be more than 3 zombies that can be human in 1 map..