Most files listed numerically go 1, 10, 11, etc. the trick is to put a zero in front of the single numbers, i.e. 01, 02, 03, 04, etc. That way the files will run in numerical order up to 99.
If the numbers are going to go over 99 you need to put two zero's in front of the first nine numbers and one zero in front of 10 to 99.