What would be the correct amount of voltage to power an Arduino UNO, XBEE shield (with XBEE module), and five Tower Pro SG90 Servo motors?
Your best bet is some battery source and a regulator to put it down to 5V (it seems that all your gear is more or less 5V powered). It was already commented that some onboard regulator could need 7V, for example.
I would, however, worry more about your current budged, powering something like 20 servos will need probably more than 1A (check your datasheet)