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:
The ultimate and unheard of combat style that combines the power, skill and drunkenness of the pirates with the silent, deadly assassination skills of the ninja.
''have you heard of Shadow Pirate Ninjitsu?'' ''No, no one has, but it sounds awesome!''- Conversation overheard before both where quickly silenced.