In any martial arts fight, there is only a finite amount of ninjutsu available to each side in a given encounter. As a result, one Ninja is a deadly threat, but an army of them are cannon fodder.This trope is very common due to the numerous storytelling considerations fueling it. Drama thrives on conflict, and having the few put up a fight against the many is basically a free conflict coupon thats automatically viable during any few vs. many confrontation. Why have the superhero team curb stomp the villain if you can make him powerful enough to force them into Teeth-Clenched Teamwork? Why have the dozens of Mooks club The Hero unconscious three seconds into an encounter if you can let him take down seven or eight of them before he collapses, to show how much of a Bad Ass he is? That would be letting some perfectly good dramatic tension go to waste.As the number disparity grows larger, another factor comes into play—theres strength in numbers, but also anonymity, which in fiction is a crippling weakness. Characterization is a precious, rare resource that is difficult to set up, which means most characters are not going to get any. Since characters often travel in homogeneous packs in terms of characterization depth, the larger a group is, the less characterization its members probably have.
via Main/Conservation of Ninjutsu – Television Tropes & Idioms.