Answer:
Obviously it depends on the location and the size of that branch's "stable" (cars on that branch's books). My agency, which is close to the airport, rents an average of 10-15 cars a day. We have room on the lot for about 20 vehicles, but they come and go as people bring back returns. Walk-in customers may add to that, and there are always reservations that don't show up.
Airport locations will do a higher volume business, and their inventories are much larger, often in the low hundreds.