hop of those help? There is no hardcoded limit in ejabberd. All depends on your use case and your MUC configuration. You can even use MUC without broadcasting presence to other members. This is defined as options muc#roomconfig_presencebroadcast in XEP-0045. As such it can behave much like Pubsub, but taylored at a chat use case.
XMPP Multi user chat setting avatar image for the group chat