I have 6 approved costumes, but you have the same number with the appropriate first 2 letters. For example, I have 2 different officer costumes, they are both assigned ID-4584, I have 2 Stormtrooper costumes, they are both TK-4584, an Imperial crewman is IC-4584 and a TIE Reserve Pilot that is TI-4584.
So Basically, you can have as many different variations of as many costumes as you want and you will always have your 4 or 5 digit number with the proper 2 letter designation.