Can you tell me how many amps would I need out of a 480v transformer to startup a 50 hp AC motor?

There are many condition to be met in sizing up your transformer to start up a 50hp 3phase ac motor.

1. the starting kva of your motor. (per NEMA code Letter Designation)
2. the voltage sag during motor starting should satisfy the 10% drop requirement.
3. the selection of starter (FVS- full Votage Satrater, RVS- reduce Voltage Starter)
4. the transformer must be sized to its short circuit capability

I can understand from your question that the transformer output is 3 Phase of 480 V.You have to connect an motor of 50 Hp to the same source.
Now I shall suggest you to check the motor characteristic curve which tell you the motor performance under load condition,Based on this you can choose the rating of the fuse & starter type .Typically for 50 Hp motor the fuse shall be 100A.