I think a lot of them never knew or it would be hard to calculate
from what little my father ever spoke about it, during his time In Vietnam a lot of the fighting happened at night or from/in heavy cover, where you would be firing at movement, or the sources of muzzle flashes
they wouldnt know how many had been killed until daylight or they secured the area I imagine in situations like that when you have half a platoon opening up on muzzle flashes or movement near the source of fire, which would be similar in trenches as well, its more how many the unit killed then being able to attribute most of them to an individual soldier
........the ghost in the machine...
IGN: Fox, FC: 5344-2646-0982