A fraction that would hover at less than 1%.
Consider: Not everyone in classical antiquity serves in a battle. In fact, for most of antiquity, military service was part and parcel with having enough money to be able to afford equipment befitting a warrior. If we believe Polybius, at the eve of the Second Punic War, the Roman Republic and Allies had a bit over 760,000 men who were eligible to serve in some capacity or another, and even with the huge armies being thrown a around and the losses suffered, they didn't raise that many troops.
Then, once you're in the battle, you have to kill someone. However, given that most of the killing is done in the post-rout chase, you probably won't get a chance to do that unless you're in a position to run down the enemy. That is going to be restricted mostly to cavalry forces (a tiny minority), or the lighter armored troops, assuming they don't get swatted aside early on in things.