Hi,
yate is fully opensource, there are modules which are not opensource which are mainly related to Enterprise/Carrier solutions.
If you want a simple PBX use Freesentral, which is also opensource and developed by null team. As I know there is no commercial version of Freesentral out there.
We are running yate in productive, with hunderts of concurrent calls, with no problem. So I guess 20 clients are no problem.
You also can use the mail group, the wiki page, irc channel or use the search function of the forum.