I guess you're talking about something like Twitch? That's certainly a use case but a rather extreme and rare one. Among public IRC networks I believe QuakeNet has the record with a peak of ~240k users in total on the entire network (that was a long time ago). Channel record on QuakeNet was ~10k users. These days it looks like only two IRC networks have >30k active users in total (according to http://irc.netsplit.de/networks/) - freenode and IRCnet.