What dleong said!
I have small adhesive backed numbers (ie 1, 2, 3, 4, etc) on the bottoms of my floorplates of the magazines, for the pistols I use for IPSC competition. Most well stocked Hardware and Department Stores will have something like that, you can use. You can also use a Paint Marker to put a number on the bottom of the magazine floorplates. Any color but Black! LOL