I have made a multiplayer area counter using the complex trigger , switching on whilst any enemy in the zone and switching off when all the enemy have left the zone.
At the moment I have only been able to test it with AI by ticking the check entities box in the complex trigger, the mission ran for over 2 hours in that time I estimate over 300 aircraft entered and left, the counter remaining stable.
if you want to check out the test mission let me know.