Until they are no longer needed in the vehicle, the safety monitor with his thumb on the kill switch is one per vehicle. Waymo has built out remote monitoring infrastructure, and in car intelligence that enables one remote monitor to cover a large number of vehicles. The vehicles ask for help when they are unable to confidently make a decision about how to proceed. The remote monitors never actually steer a Waymo vehicle. They pick from a set of choices, or they decide to take the vehicle out of operation.

In the short to medium term, safety monitoring will be the limiting factor.