The number of channels you can join is determined by the server you connect to. Some servers only allow 10 channels, some allow 20, some allow 60+. Either way, it has nothing to do with mIRC I'm afraid, therefore, not much we can say except try another server!
You can see how many channels the server allows by looking at what looks like a bunch of rubbish text when you connect to the server:
"NOQUIT SAFELIST MODES=6
MAXCHANNELS=20 MAXBANS=100 NICKLEN=30 TOPICLEN=307"
It looks like that. The bit in bold tells you how many channels you can join.
I guess you could request the server administrator to increase the amount of channels, but the chances are small. The server admin should be named in the Message Of The Day (MOTD) - to view it, type
/motd.
Happy chatting
Regards,
