Do Tesla cars only have one gear?
Yes, all currently produced Tesla cars have only one gear. (Although they have experimented with a two-gear system in the early days.)
Tesla don't even have a neutral gear nor a reverse gear! How does this magic work, you ask? Ah, happy you asked.
The electric motors in a Tesla are connected to the wheels via a fixed reduction gearbox. The "reduction" gearbox "reduces" the speed of the (incoming) motor axle to the speed required to drive the wheels on the outgoing axle. This is needed because the optimal speed of the AC (alternating current) electric motor in a Tesla is much higher than the speed required to make your wheels turn around. In fact, the axle speed is reduced by almost a factor of 10. That means that when a Tesla motor rotates 10 times, the wheels of a Tesla do one single rotation. At the same time, a reduction gearbox increases torque on the wheel axles, giving you that extra pulling power for the quick getaways Tesla are famous for. Especially if you know that an electric motor delivers its maximum torque from 0 (zero) rpm onwards. Vroaaammm! I mean, Bzzzzzzzzz! ... and you're speeding before you can say Chihuahua!
Each Tesla motor is controlled by something called an "inverter". The inverter turns the 400V DC (direct current) from the Tesla's battery into a modulated AC current. By doing that, the microcomputer in the inverter can accurately manipulate speed and rotational direction of the motor. Therefore, there's no requirement for a reverse gear. The inverter simply makes the motor go forward or backward. Side note: Theoretically, a Tesla could go (almost) as fast in reverse as in forward. In practice, the speed when reversing is software limited.
At the same time, when no electrical current flows through the motor, it rotates freely. Hence, there's no requirement for a neutral gear either.
So now that you know that, you also realize that the DNR (Drive, Neutral, Reverse) gearstick in a Tesla does nothing else but give a signal to the computer in the inverter to have the motor turn in a forward direction (Drive), send no current through the motor at all (Neutral), or have the motor turn backwards (Reverse).
All of that with a (single speed) fixed reduction gearbox. Neat, huh?
The current vehicles made by Tesla Motors have only one speed ratio between the drive motor and the wheels - which is what many people think of as "first gear" or "fifth gear" on a conventional transmission. There is no way to "shift gears" on a Tesla.
The cars do have multiple gears, in the engineering sense: in the fixed-speed reduction gearbox on the motor output(s), in the differential mechanism between that gearbox and the output axle shafts... and other gears inside all sorts of other vehicle components like the windshield-wiper mechanism, power windows, etc.
Long: You've asked the wrong question, but I can see how to got there and can straighten you out.
Currently, all EVs I'm aware of are using hub motors. That is, the motor is in the wheel of the car. Because they are electric motors, the speeds they can attain is limited only to the windings on the motors, and how those windings are patterned. You can apply more and more power until you hit certain upper limits, because you aren't adding more fuel, you're changing the state of a potentiometer.
How winding patterns affect electric motors, I don't know.
The motors are centrally mounted, and apparently do have a transmission, though it isn't quite the same design as an ICE based drivetrain would have. Still, yes, Tesla's only have 1 gear in terms of shifting.
Are you asking whether a Tesla has a gearshift control like conventional cars, or whether there are speed reducers in the powertrain?
You might be able to answer your own question if you have ever seen a variable-speed power drill. Where is the gearshift know on the drill? It has gears in the front end to reduce the motor speed from a couple thousand RPM down to whatever speed you need for driving screws or drilling holes.
A Tesla (or any electric car) could have a gears but they generally don't need them.
Electric motors have a much wider, flatter torque curve than combustion engines. That's why combustion engines have gear boxes btw.
Tesla cars have one gear, a simple reduction gear. For the car to run in reverse, the motor runs backwards.