That's roughly as many employees as Apple had in 2005, a company that was making laptops, desktops, servers, wireless routers, portable audio players, and entire operating systems.
I doubt a scrappy "startup" like Uber can beat companies like Toyota, Volkswagen, GM, Google or even Apple to market with a viable, certified self-driving car. They have, at best, a few billion left to dump into that venture. Those big companies could throw in ten times that if they wanted to.
Maybe someone needs to disrupt Uber. Go Lyft!
Other than engineering they also have a lot of people going around mapping cities, QAing those maps etc.
Ps... you'd crap your pants if you knew how many contractors they have doing operations work. Their mostly in low cost counties like the Philippines, and Brazil. Nothing about support scales with growth.