SLA Marshall estimated that during WWII:
In any given body of American infantry in combat, no more than one-fifth, and generally as few as 15 percent, had ever fired their weapons at an enemy, indeed ever fired their weapons at all. Source
Marshall's findings have been criticized for its methods. The military responded by desensitizing recruits to the idea of killing. Rather than use euphemisms or candy-coat the proposition, trainers used the word "kill".
Ratios of Fire were estimated to be 55% in Korea and and 90% in Vietnam. Source
Absent a campaign of "desensitizing" estimates of 10%-to-20% "Ratios of fire" are logically defensible.