How many output amps is a 75 kva transformer?

For single phase, KVA = (line to ground) * (phase current).

A 75kVA 480 to 208Y/120 volt transformer is a fairly common transformer. I assume this is the type of transformer you are referring to. 75k / 120 = 625 Amps.

As an FYI, the 208Y voltage is the line to line voltage, which is equal to (phase 1) - (phase 2), where the phases are separated by 120 degrees, thus (phase 1) * 1.732

For three phase, kVA = (line to line voltage) * (phase current) *(sqrt 3),

75k / 208 / 1.732 = 208 Amps.