How many letters can a power supply light?

It depends upon what letters are chosen.  Each letter has a different power draw, so that is why we calculate it for you.  As a basic guide, 7 or fewer letters/numbers in a row normally use less than 1 amp.  More letters in a row normally calls for the 3 amp power supply.  More than 22 letters in a total sign will often exceed 3 amps, so one or more additional  power supplies may be needed.