If you want to use a servo with your arduino, you can be lucky that it will use little enough power to be driven by the on board voltage regulator. But it can still be a bad idea, because of the sudden current spikes when the servo motor moves.
If you can’t, or won’t, use the on board 5V voltage regulator, you need to connect it like this.
Here the 9V battery is connected to the arduino’s VIN pin, but this is not needed, it can be powered by usb or another power source without any problem. Only thing important here is that all the GND are connected to each other.
But on the picture the 9V battery both drives the servo motor, and the arduino through the VIN pin. But between the battery and the arduino there are a 7805, which regulates the voltage down to the 5V a servo motor can handle.
It is now instead the external 7805 that are driving the servo motor, instead of the on board.
Like this you can add as many servo motors as you want, only remember that a guideline is 1A pr servo, so if more needed, you need to put a 7805 more in, and make a parallel circut for that too, just like it is made for the servo motor in the example.
The picture is only a slim example on how the wiring is done, and when you build it, you should add a 100nf to 300nf capacitor between gnd>in and gnd>out