Tesla always speaks about one motor on its Rear Wheel Drive Model S car. But every time I see a picture of the rear axle as in the search https://www.google.pt/search?q=tesla+bottom+motor&safe=off&tbm=isch , it seems it has got there two motors.
So how many motors does it have?
What you are seeing is the motor which is an AC motor. The center of it is a simple gearbox. Because an electric motor can be throttled and can run up to a speed far beyond what a gasoline driven engine can run, there doesn't need to be a transmission. Then to the other side is a three phase tri-pole inverter which converts the DC battery power into AC power to run the motor. (You may have something similar which you use to power a laptop or other A/C using devices in your vehicle, but this one is on a whole different scale.) This How it's Made Dream Cars episode (S02E10) explains how everything is put together.
It is a single motor.
A good source of information is here. About 15 seconds in, they describe that one side is the electric motor. The other side is the power electronics. The middle is the gearbox. I believe from looking closely at the video that the slightly smaller side (the right in the picture in the question) is the motor.
