There are ten possible digits (0 to 9).
If they can be repeated, you're looking at 10 * 10 *10 * 10 * 10, or 105
This means 100000 possible zip codes if repeatable.
If not, you're looking at one less each time:
10 * 9 * 8 * 7 * 6
90 * 56 * 6
540 * 56 = 30240 zip codes without any repetitions.