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Well it's only really kept track of when it comes to pilots which are called aces, there is loads of aces knocking around, snipers are the other one I can thing of.

But as for how many a single soldier has killed Simo Häyhä is an interesting one he was a Finish Sniper.

Simo Häyhä had 542 confirmed kills over the space of 100 days, the Russian's tried everything to deal with him, counter snipers, artillery strikes.

He did a few weird things like sniping with snow in his mouth to hide the steam from his breaths in the cold air he was sniping in.

Eventually he was hit by an explosive bullet which blew out the left side of his face, he didn't die, he in fact lived on to die in 2002, as the war ended shortly after his injury.

So that's the only one that comes to mind in my period other than fighter aces. Simo Häyhä 542 confirmed kills on Russian soldiers in 100 days.