This must be a trick question?
The torque curve is pretty flat until it reaches a threshold speed whereby it declines by speed. Namely the motor efficiency drops rather steeply with higher speeds. The decline in torque is faster than the increase in RPM and thus power drops and efficiency decreases and motor thermal energy increases.
So in order to have a faster car, the RPM must be dropped below the threshold, which means the gear should be increased. Thus we need at least two gears.
If you provide me the spec sheet for Tesla's motor, I will try to work it out.