The conservation of ninjutsu is the name given to the law of narrative that says the number of enemies is inversely proportionate to the power of each enemy. More simply, the more Ninjas there are in a given fight the less of a threat they pose. Everyone knows that ninjas are highly trained assassins whom often possess magic powers. Any single ninja is likely to sneak past dozens of guards, effortlessly scale walls, and glide past any security measure with contemptible ease in pursuit of their target only to be stopped at the last moment by the hero and his cohorts. Yet, any room full of ninjas is a minor annoyance on the way to the final boss.

This trope shows up in a variety of places. The Teenage Mutant Ninja Turtles can take down twenty foot soldiers easy, but lose to the shredder. Naruto is the living embodiment of this. His primary technique throughout the series is making clones of himself with the drawback that it divides his chakra up evenly between them and true to the rule the more of them he creates the less effective he is at whatever he's trying to do. Doctor McNinja has this as an actual element of his world with it seriously imperiling large groups of ninjas.

Of course this is hardly limited to ninjas. Take Dragon Ball Z's example where Raditz beats the crap out of the protagonist and his frenemy to a stand still without breaking a sweat. Later the Saibamen, which we're informed each have the same power level as Raditz, are defeated by freaking Krillin. It's generally a safe bet that when largely homogeneous forces show up en mas they'll go down easily.

RUST IS FOR THE WEAK