Sam is right. The VRRA tries to limit to 36-40 bikes on track (depending on the track) but we will start in two waves on occasion when 2 or 3 older period smaller classes race together.
However at Barber AHRMA deals with so many people that some races started three or four classes in individual waves, but in that case there were fewer riders, maybe 15-18 riders per class. The starter waits for the first wave to clear T1 before releasing the second, likewise for the 3rd wave. This spreads the field out some. For example in the F500 race, they also ran 500GP, 500 Sportsman, and the Bears race. I think there were only 2 riders in the GP class though. (probably 50 bikes total)
I believe the CB160 race that our Chris raced in had sixty odd riders in one class and they had a Lemans start to boot.
Traffic becomes a real issue and considering different speeds and all the other factors, it keeps guys on their toes!