This Mumble Moderator modules can limit the user count of a channel. This functionality is already included in the official Mumble server but server wide only. usersperchannel. With this module one can set different limits per channel.
If a user enters a „full“ channel he will immediately be moved back to his previous channel.
Or use my fork at https://github.com/Natenom/mumo-maxusers which moves users into the default channel or root channel if a channel was removed while the user was in the AFK channel through the DeafToAFK – Module for MuMo.
An example configuration can be found here.
On our server we use the following configuration:
[maxusers] servers = limit = 0 ret = 15 exceptions = admin ;Zweiergespraech [channel_2956] limit = 2 exceptions = moderator ;Gruppentreffen [channel_2956] limit = 4 exceptions =