Like I said, 2%. This is the allowance that Grossman and his research support as being "sociopathic/psychopathic". Infantry units on average would make up close to 10-30% of an Army, depending on the force. The whole book is about the factors that can increase/decrease PTSD occurrences, but his basic point is that killing is so unnatural that PTSD is the normal part of the human reaction to killing. (IOW, everyone gets it to a degree).

He also does not say this 2% is pre-disposed to joining SF units. He does, interestingly enough, suggest that the data does point to this 2% being pre-disposed to mercenary work.