It doesn't really work like that. Some vehilcles are rented more frequently than others, while others are rented out less frequently. Sometimes you will go to a car rental lot and there will be vehicles as far as the eye can see. Other times, you may be able to count on one hand the number of vehicles that are available. There are high and low demand periods, but all the vehicles end up on rent some portion of the time.

For example, Hertz in the Los Angeles area has been sold out the past two weekends at many locations. We're talking completely sold out at Santa Monica, Venice, around LAX and even LAX airport itself. You go on their website to reserve and there is zero availability. There might be a few scattered vehicles around, but they are not taking any reservations because they are at or close to 100% utilization.

The rental companies have gotten better at "right sizing" their fleets in recent years based on expected demand patterns. They aren't perfect by a long shot, but for the most part, they do a decent job. In short, they use all their cars, it's just that the utilization may vary depending on location, model, demand, competition, etc.