on *:text:!howmany:#aribun21: {
  msg $chan there are $calc($lines(list.txt)) people remaining in the list

If you want the above ignored while the list is closed, you know which line to borrow from the other !trigger.

You can also add the headcount to other commands. For example in !join, where the current message says

msg $chan Added $nick to the list

you can replace that command with something like:

msg $chan Added $nick as the $ord($calc($lines(list.txt))) person in the list

$ord(number) puts st, nd, rd, th after the number. i.e. 2nd 3rd 11th 21st

Adding the headcount to !join will cut way down on usage of the requested trigger.