hmmmm.......
indeed, you need to a lil patient i think

. there are many good scripters here that may be of assistance to you if you give them a chance.. but for now perhaps, you can try somthing like this:
alias -l user_count {
%ucount = [ $+ $nick(#,0,a) $+ ]
;this line sets the count of all users surrounded in brackets as a variable
aline -l $active %ucount
;this line adds the user number to the listbox of the active window
}
please note this was not tested , but should put you on the right track.
-EDIT-
possible usage :
raw 366:*: { user_count }
the raw 366 code, is the the /end_of_names list . meaning it would trigger when mirc recieves this raw #
(someone please correct me if i'm incorrect on this)