Actually, in the on JOIN event, mIRC doesn't know. Not until all the names have been read. As for manually updating the IAL like gemeau suggested - that's not needed. When you join a channel and the IAL is on, it will be automatically updated. All you need to do is change the script to be trigered by the raw event that indicates the /End of names. Don'ty recall the numeric atm.
raw number:*: echo -a You have joined $2 with $nick($2,0)users.
PS: since I'm not at home, I can't test it, but if I recall correctly, $2 is the channel.