The battery voltage is one factor that determines how fast the motor spins. The other is Kv, that is revolutions per volt.
The faster a motor spins a prop, the more amps it will draw. To avoid overloading the motor, high kv motors will use smaller diameter props than the lower kv version.
The motor size, 2212 for example, refers to the size of the motor stator, 22 mm diameter & 12 mm long
How many amps the ESC can provide. If the motor + servos draw more amps than the ESC is rated for, you will burn it out.
Don't cut it too close, it's good to have a little overhead. If the motor can draw 30 amps, use a 40 amp ESC.
Each 9g servo draws about 3/4 of an amp at full load. With a BEC rated at 3 amps you can safely use 4 9g servos. You can use more servos provided you don't use more than 4 of them at one time or don't put all under full
