Medals have levels to which you can upgrade them. For example, if we're talking about the Life Guard medal, there are 3 levels available. To get the first level, you need to revive your allies 10 times. Then, to get the second level, you need 400 revives, and the third - 2000 revives.
After you get the last level of the medal, you can repeat the last level an unlimited number of times. So the number you see on your medal is how many times you've repeated that last level. If you see the number "5" on the Life Guard medal, it means you've got the 3 available levels and then repeated the last level 5 more times.