I wanna make a raspberry pi car using an RC car. I would like to know if the servo multiplexer is necessary?

Since you want to control the throttle and steering angle on the car, you will need to be able to send PWM signals to the ESC and servo, respectively. Given that you are using a raspberry pi (presumably mounted on the car), there are several ways to do this. The easiest is to use a PCA9685 servo controller and have the RaspberryPi talk to it via I2C serial. Other options are to send the PWM commands to the ESC and servo directly from the RaspberryPi using the RPi headers. See here . Finally, you could also use an arduino in place of the PCA9685; use USB to send commands to the arduino and use the arduino to output the pwm, see here.

The Donkeycar project might be a good place to start. You can look at that project for how to set up PCA9685. https://www.donkeycar.com/. That project also includes a web server that runs on the car; so you can send the control commands over via a web browser - this could easily be extended to another http client. You can also control the via bluetooth. This is a presentation on the anatomy of such a car https://www.youtube.com/watch?v=pq6hiZEdirE

Shot answer: no

It depends on how you want to control it.

main function was to convert radio signals to PWM signals

I don`t see the reason of converting radio signals to PWM, as long as it is RC car and you have remote control, you can control it directly using RC.

Possible solution:
If you wan`t to control the car (DC motors) using program running on RPi, you need 2 things:

1. H-bridge (motor’s spinning direction can be controlled by changing polarity of its input voltage) like L298N (a dual-channel H-Bridge motor driver)
2. any Raspberry PI library for PWM (for ex. RPi.GPIO) for controlling speed of the rotation by setting duty cycles.

The thing that 'convert[s] radio signals to PWM signals' is usually called a receiver.

A servo multiplexer such as https://www.pololu.com/product/721 switches a bank of servos from being controlled by one source to being controlled by another source.

If the raspberry pi is mounted in the car and will be the only thing controlling it, you don't need either, but you will need something to allow the pi to control at least two servos. There are boards to do this, many based on PCA9685 chips, though for a simple car you only need two channels and could do it with a few components.

If you do want to be able to switch between the original RC controller and the raspberry pi on the fly, then you would need the multiplexer to do that, but it would only work if the controller has a spare channel to do so.

This assumes it's the sort of RC car with a receiver, and a proportional servo for steering, and a ESC for motor control, both controlled by PWM. Cheaper cars with integrated electronics will use an H-bridge to directly power the DC motor and often a second H-bridge for steering, where it only has left, right or straight ahead control. In that case a robot board with two H-bridges will suffice.