It's the number of battles he's been in, if it were droids killed I'm sure there would literally be hundreds of marks & no room for new ones. lol
Now for the nit pick regarding the arc troopers..., there aren't really phase II versions of Echo & Fives, they are in fact phase 1.5 as the helmets are neither phase I or phase II..., they have the flared out spike on top of their helmets like the phase I clone trooper helmets plus the nose section of the helmets is more pointy than that of the phase II helmets, I was told another difference but can't remember what it was off hand. I was done buying clones as I felt I had enough but when It was pointed out to me about these really being more like phase 1.5 well, I don't have any of those so I preordered them. lol