yep, you're right. What happens is this:
When you initially join a channel, mirc thinks you're the only one in there. Then, it automatically runs /names, and uses the response to fill the nicklist. To see how many people are in a chan when you join it, or optionally how many ops, voiced nicks, etc., you need to respond after /names is done. For that, you use:
raw 366: [Your Nickname] [#channel] End of /NAMES list.
so,
raw 366:*: { echo $2 There are $nick($2,0) people in here. }
You can use $nick($2,0) to return the total # of users, or other variations to find out other info. check /help $nick (Nick and Address Identifiers section) for more info on that.
-Venoman