It depends. Some of the girls I know say that a "busy" day for them is 5 customers a day in a 10-12 hour shift.
But depends how popular they are. Alot of times, i can book in a girl within even half an hr of arriving. So I doubt alot of them are booked back to back.

Regular Japanese girls I see, even if I am last one for a 12 hr day, they still give me the same great service. But I'm not sure if its because I'm their regular, so I cant say if same service will be given to a customer who isnt a regular or a customer they dont particularly like.