If you would ask the boys how many people they think they killed what would they say?
Considering the adrenaline rush of the out-of-body, crazy madness of the moment they were in, their detached minds distorted their perception and skewed reality. It probably felt like they killed at least 40-50 people. At least, it viscerally felt that way to them. While in the library, they were literally reloading weapons, walking around and pointing and shooting under the tables randomly and then reloading again. They heard cries and whimpers and people scared and in pain. They saw feet and legs under tables, saw people spasm after being shot, saw a whole lot of blood everywhere and on themselves, smelled gun smoke. The combined perception of all of those sights/sounds/smells would lead them to believe that they killed many more than they actually did.