I hate to put it like this, but the answer is: it depends.
For our conveyor, we're using only 1 window motor. With our roller, it gets the balls through 40" of conveyor in 5 seconds. This is acceptable to us, as the motor has more than enough torque and the speed is 'fast enough'. We also are trying to conserve weight, so more motors are impractical right now. (Our drive train + pneumatics + electronics board came in at 65 lbs

)