Is Jamulus peer to peer?
JackTrip also supports a serverless peer-to-peer (P2P) model, where every client streams their audio directly to every other client. This is used by JamKazam and other online jamming solutions. If you have to open a port in your firewall, you are likely using P2P.
How many people can use Jamulus at once?
It supports up to 50 simultaneous users. And it's free. The following pages cover the various aspects of using Jamulus: installing the software.