OK thanks. There are so many variables. At a training airfield where I once worked it was not unusual to have 6-8 in the circuit with several joining, all talking to the Tower controller. They were all VFR so they were basically looking out for themselves. However, try 8-10 IFR on the frequency when you are going for 2.5 to 3nm final approach spacing. That can be busy. However, in controlled airspace IFR flights are usually following strict procedures with highly professional pilots which helps considerably. I believe the RAF had rules about how many aircraft could be handled at once but when I worked in UK Civil ATC no such rules applied.