The game was initially capped at 64 players, but due to various factors the servers were hard capped down to 40 by the Devs.
The servers will accept a player limit of 40 by default, no mutators need to run to get to this value.
if you try altering configs + server bat files to give you more, it just runs at 40.
The mutator for max players overrides the hard cap when it starts allowing the server to host more than 40.