As dismal, atomized, space-inefficient as it is, it would be surprising if it wasn't a two seater: Most taxi rides are two or fewer people and they're already pumping out enough 5 seater autonomous-ready vehicles to saturate the market for the rare taxi ride that needs that.

If autonomous cars can do 5 times the usage, that will over time, make reductions in cars owned and increases in taxi ride demand.

How it goes at first seems likely to generate lots of negative reactions. There will be areas of excess supply as people first try it out meaning very few fares and not worth having lots of miles driven on your car for insignificantly low fares. There will be areas of excess demand meaning long waits putting customers off. There will be stories of drunk person(s) having been sick in car.

Getting through all this to the point where fewer cars are owned will be difficult to manage. Is Musk underestimating difficulty and overselling benefits? Possibly, I am not sure. Maybe it gets there eventually?

Part of what I was getting at is that they already have a lot of momentum in manufacturing 5-seaters, so there's unlikely to be a time in the next 20 years when they suddenly have way more 2-seaters out than 5.