Servos are very “noisy” on the power supply. Keeping the RPi and servo power separate is highly recommended.
Most servos are designed to run on a 6v power supply, but will work (with reduced speed and torque) at voltages as low as 3v or so.
The current requirements of a servo will depend on the model. Some small servos might have a stall current of 500ma at 6v, while larger servos might have a stall current of 3A at 6v.
If you use a wall power supply to power the servos, it would be advised to use one rated for the stall current of all servos you intend to use. For example, if you are using two servos, with stall currents of 750ma and 1500ma, you should use a power supply rated for at least 2250ma (2.25A) continuous. To prevent brown-out or overcurrent conditions, it would be good to oversize the power supply by at least 20%. In the example above, a power supply rated for 2.7A or greater would be ideal.
Make sure you use a regulated power supply. A transformer with a DC output rated for 6v 2A might have a no-load voltage of 9v (too high for servos), and a 2A voltage of only 4.8v (reduced speed and torque). A regulated power supply should have a constant voltage of 6v, whether at no load or at max load.