I'd still want my car, and I wouldn't want it just driving in circles for hours. Less spots could be necessary in downtown areas, since a car can drop you off and park a few miles away. car ownership isn't going to disappear with self driving, it may reduce, but plenty of people have use cases that a taxi model isn't great for.

Yeah that's my point! You could calculate the amount of needed cars and have them in circulation, and they could park themselves in some underground garage somewhere when usage is low. It could be just one big fleet of self driving taxis, self-driving Ubers.