The creator of the custom campaign specifies everything in the map (tanks, SI, CI, ...)
We are using SourceMod plugins to adjust these values (numbers of infected, health, ...)
Our adjustments are also based on the amount of players connected to the server.
So if more players are connected, more infected will come.
You can play the custom map alone just on your client (without server) to see how a custom campaign is configured without any server plugins.