What is the rated current when a step down transformer is 50kVA and 120v in step down to 24v?

The VA of the transformer is the product of the amps x the volts. V x A = Watts. 50kVa is equal to 50,000 watts. It holds true on the primary and the secondary. The formula to use is I = W/E. Amperage = Watts (V x A)/volts.

The above answer will determine the rated currents of the primary/secondary windings. But the actual currents (which shouldn't exceed their rated values for any length of time) are determined by the load. For example, with no load, no secondary current will be flowing, and the primary current will be reduced to its magnetising current value.