Jitsi "Start a call" button doesen't work with more than two partecipants

Jitsi “Start a call” button doesen’t work with more than two partecipants.
What I can do?
I must use a personal server for more than two user chat?

I have used google chrome.

Check advanced section of quick install guide

I have used your public server. Often i have seen another uninvited user whoose name started with Fellow. Is the normal behaviour of a session?

I have resolved with a private installation. It is a very good project. Now we are going to do some experiment of remote lesson in this period of closed schools. It is a very useful resorce.