I have found that it is still "comfortable" to fly at events with 3-4 other airplanes in the pattern with me. With 5 or more other airplanes in the pattern it just isn't fun anymore because I end up spending the entire flight avoiding traffic.
I would suggest you allow up to 6 planes in the air at a time. You should also require all pilots to have a spotter with them on the flight line, and be sure that all of the pilot stations are close enough together that the spotters (and pilots) can communicate with each other easily. At the pilot's meeting make sure you stress to all the pilots that they should let everyone else on the flight line know their intentions by announcing taxi, take off, low pass, slow flight, emergency. gear pass, landing, etc...
It makes flying much safer and more fun when you don't have to guess what other airplanes are going to do.


When are you thinking of having this event?