rpt007:
You need at least 12V from an external power supply for the motor shield.
The amps are determined by the connected motor
The motor shield can run on a wide voltage range 4.5 to 25 volts.
The MOTOR should be selected not to exceed the current capability of you driver. (0.6 amps for the l293d or 1.2A peak).
The power supply should be capable of providing the current needed for peak draw.
If you have a motor rated for 5v then do not run it at 12. Run it at 5, just not from the arduino regulator.