What cvar or mod is required to increase max players in TF2 MvM

I have noticed that a few community MvM servers allow more than the default 6 players per team. What cvar needs to be set or what mod needs to be installed on the server to set the max allowed players to a higher (or lower) number?

I would prefer setting a variable over a mod, but will go for a mod if there is no stock functionality for changing max team size.