How do i set max. connections on session?

How am i supposed to set the maximum connected users per session?
However this leads to another problem. Say for example that you have 20 max. connections session that is full with 19 subscribers or publishers and 1 moderator. Suppose the moderator leaves the session for a while and while he is away another subscriber connects. Now there would be no chance for a moderator to log in, which is really bad.
Is this a openvidu server configuration or do i have to manage it using my backend? How do i do it in the later case?

Every time one participant wants to join to a session it has to get a token from your backend.

You can control who is able to join controlling who is able to get the token from your backend.