How can calculate amps for 12v 50w halogen lamp?

The formula for current is Amps = Watts/Volts. The lamp itself would draw 4.16 amps. Since the voltage of the lamp is 12 volts there is a internal transformer involved in the fixture itself. It doesn't matter what the input (primary) voltage to the transformer is, so long as it meets the manufacturer's specification as to the proper voltage to operate the fixture.