You will not need as many total cars as they can be shared and most people do not use them most of the time anyway. Think health clubs and Internet service provided by telcos. They can be oversubscribed by designed because providers know that not everyone will use simultaneously.

You need less cars to serve the needs of the currently-in-motion population than for everyone to own their own car. The idea is, a smaller percentage of cars are parked at any given time, and can meet demand with fewer cars. Fewer cars, less parking required.

Well, they wont have to take up prime real estate. You could move them somewhere cheaper to build, and in a dense arrangement.