All the organizations promoting self driving cars envisage the end of the private automobile, so yes, there will be far fewer cars. Today, most cars spend more than 95% of the time parked. Think of all the land that frees up. The key to getting the benefits of self driving cars is legislating an end to people driving their own vehicles. The cost savings in terms of car payments, insurance, environmental impact, freed up land would be massive. You would only pay for the transportation you use.