I have no idea what the evidence really says on this topic, but I suspect the claim is more likely to hold true in the past era of huge conscripted armies, than in the modern error of small highly-trained armies (and small guerilla armies, which don't have large numbers of conscripts).
This link is annecdotal gibberish.
https://en.wikipedia.org/wiki/S.L.A._Marshall#Controversy_af...
People have looked for the evidence for his claim but can't find it. I've had people repeat the claim to me and cite Grossman or Marshall, but when I asked them to give a specific source for it they couldn't.
Dave Grossman reportedly cites someone named S. L. A. Marshall who made the claim that only a small percentage of infantry ever fired their weapons, to support the idea that most people have some natural reluctance to kill in battle. I just want to point out that there is zero evidence for this claim: https://westhunt.wordpress.com/2014/12/28/shoot-to-kill/