Your questions have nothing to with Jabber. You have a basic misunderstanding on how all this works.
Conference calls don't "run on Jabber", it's just a telephony client (like a normal telephone).
Per default, audio conferences normally are hosted on CUCM (and there, per default the max. participants are 4 per conference call, but that can be edited in CUCM).
The same for participants in the chat room. Has nothing to do with Jabber either, because Jabber is not "hosting" the chat room. It's the IM&P server.
If you want to know any limitations, then check out the documentation for the servers (like config, install, admin guides, ...)