I am sorry that this question is a bit vague, but…
Theoretically, can an OpenFire server support tens of thousands of subscribers subscribed to hundreds of publishers?
from 2006 mentions some architectural issues that might keep an XMPP server from scaling multi user chat and Publisher-Subscriber messages. From the discussion it appears that this constraint would prevent a scenario like the one I describe above.
thanks for any addtional info you can provide.