A law of the Universe, that states that there is a finite amount of ninjutsu available to each side during any given battle. If there are many ninjas, they are all awarded a fraction of the total ninjutsu, or 1/N where N is the number of ninjas present in the fight.
This is the reason why hordes of ninja warriors will be dispatched relatively easily, whereas the lone ninja is almost unstoppable.
Practical applications of the law of Conservation of ninjutsu:
Sir! There's an army of ninjas heading this way!
Don't worry, we can take them.
Oh wait... it seems they've given up, sir. There's only one guy.
GOD DAMN IT RUN!