The NASA X-57 uses 12 electric motors for takeoff. Does it need 12 control levers or just one?
The need for throttles is to provide a linkage to the carburetor/throttle body in the age when everything was mechanical. The throttle in older airplanes directly controlled the air-fuel mixture allowed into the intake manifolds. If electric motors were used, the throttles would have been attached to mechanical rheostats. 

In the age of computer controlled engines/motors, this direct linkage is not necessary. You would essentially just tell the computer how fast you would like to go. The system will take care of the rest.

In most cases, the throttle is retained in the cockpit to provide the human pilot with a familiar interface with the computer. Honestly, in an airplane, where throttle settings and changes are more stable and predictable, a knob or a couple of buttons would suffice. FADEC controlled engines/motors are pretty standard now. The method of inputting your desired controls in a fly-by-wire system is arbitrary. It is just a matter of comfort level with the controls. We might even get to the point of a true GUI cockpit one day.

In the case of a multi-engine electric airplane, you would want at least two controls to account for differential thrust when desired. This is not something that you would absolutely need. But, it would be convenient. 

Some very complicated aircraft like the V-22 Osprey rely on the pilot not having direct control over the engine-prop systems. It’s absolutely necessary for a computer system to coordinate the two engines during VTOL operation. The same problem would present itself in a quad-copter large enough to carry human passengers.

As an aside, I vividly remember the first throttle-by-wire car our family ever owned. It was a Hyundai Santa Fe. Though, not difficult to drive. The throttle response took some getting used to. Also, modern electric cars with multiple motors have only one throttle.