Well, part of the problem is that even though Proximities are useful, they are also very limited and don't handle certain things as well as they should. Really, IMO, the best way to do it would be to run the clientsearch check that you can get from the Rampage script that Kayen posted here:
http://www.eqemulator.net/forums/showthread.php?t=27850
You can do distance checks and even count in real time how many players are in range.
Then, probably what I would do would be to assign temporary qglobals to each person that enters the proximity. And, have it set an incrementing value for the qglobal so that it starts at 1 and goes up just like the enter_count variable does.
Then, you can use the client search code from the Rampage script to look for players and check their qglobals. If their value is greater than 6, it kicks them out.
Though, this would mean that once the limit of people are reached in the proximity, they would be the only chars that could do that event until their qglobal ran out.
You could also use a combination of that script along with the player.pl and have the player.pl do an EVENT_EXIT_ZONE or whatever the event for leaving a zone is called. And, when they leave the zone (die/gate/whatever), it deletes their temporary global and makes it available to the next person to enter.
There really are quite a few options depending on how creative you are about it and exactly what you are trying to accomplish.
If you wanted a simple way to handle it, you could just set the zone in the zones table to only allow a max of how many players you want to limit the encounter to. But, if you are only wanting to limit it for certain NPCs and not the whole zone, that wouldn't really work. Again, it depends on exactly what you are trying to do. I figured I would just give you a few ideas about things to consider. With enough creativity, a ton of stuff is possible that you wouldn't normally think would be possible.