Actually, you take that number, PLUS the number of empty Uber cars, PLUS the number of empty taxis that would not have been empty if there wasn't Uber...
That just isn't true. They often do drive around and look for fares or go back to a place where they can collect a fare from where one did not exist. Example you pick up Passenger A in dense urban center drop them off in suburb no fares exist there. You need to drive back to the urban center to pick up another Passenger. You have worsened traffic and created extra carbon pollution.