If you're asking how many aircraft can be on final in a procedural approach environment it's limited by the weather conditions. As a TWR and APP controller you can't clear a second aircraft for the approach until you can see the first, and there is reasonable assurance that a normal landing can be made. So usually you need 3 or 4 minutes between arrivals in most conditions. If you have to protect the missed approach (below circling) then you need more like six minutes as you need to wait until the first aircraft is on the ground before the second can be cleared for the approach. Aircraft can be instructed to join the initial approach and maintain a level above preceding traffic provided there is enough spacing between them to issue an approach clearance in time to allow the aircraft to descend on a normal profile and that an approach clearance is issued prior to being established inbound