200 enemies looted.

As mentioned above, you will always be able to loot an enemy solider by pressing X over their dead body. There are more than enough enemies within the game, so do not worry about this too much as it should unlock near the middle of the game if you loot casually. Remember, you can always use your Instinct ability to see which enemies you have not looted yet. Their bodies will glow yellow in comparison to those who have already been looted.