You need to determine the voltage range of the servos. You will supply the power
to those directly from the power source ( battery ) rather than through the arduino. Servo
voltage varies from 5 to 6 to 7 volts or more. A servo rated for 6V will work better
at 6V than at 5V.
The arduino power input is ideally between 7 and 10 volts.
It depends how long you need the batteries to last.
If you connect 4 x 1.5 volt batteries or 5 x 1.2 voltage batteries in series, you get 6 V.
You can supply this directly to the servos +V input and directly to the arduino input plug
where it will be regulated to 5V or 3.3V for the arduino itself.
Another plausible alternative to consider is two LiPo cells in series which is about 7.4 V.
You can get a 6V voltage regulator to supply the servos from this.