A single servo under load might draw more than 1 Amp. An Arduino running off of USB only has about 400 mA (0.4 Amps) of 5V to spare! Run off a 7-12V supply it has about 900 mA to spare (0.9 Amps). Sometimes a single small servo will work, even on USB, but you can't count on it.
My understanding of servo power requirements is that the motors are specified at 6V and can work at 5-7V. The Arduino regulator needs 7V minimum. If you can get a hefty 7V power supply you can use it for both the Arduino (Vin or power jack) and servos. If you can get a hefty REGULATED 5V supply you can use that for both Arduino (5V pin) and servos.
There are three options:
1) If you can get a hefty 7V power supply you can use it for both the Arduino (Vin or power jack) and servos.
2) If you can get a hefty REGULATED 5V supply you can use that for both Arduino (5V pin) and servos.
3) If you can get a hefty 6V supply you can use that to power the servos but you will need a separate power source for the Arduino (USB or 7-12V in the power jack).
In all cases, 'hefty' means about 1A per simultaneous servo. If you have 10 servos and only one moves at a time you can probably use a 1A supply. If you move all 10 together you may need a 10A power supply.
Most multimeters can measure current and many have a special 10A jack for measuring high currents. You put the meter between the load (servos) and the power source.