The inverse ninja law is a media trope regarding not only ninjas, but any character type that is shown to attack in massed numbers, such as soldiers, robots, daleks, or vampires (but not zombies). It states that the threat level of any number of ninjas or other whatsits is inversely proportionate to their numbers. Therefore, if you're attacked by a lone ninja, you're in trouble, but if you meet an army of ninjas, they're going down.
The rule of cool provides a potential explanation for the inverse ninja law: simply put, it is always cooler for a combatant to win a battle while at a disadvantage, and numbers are a clear advantage. Ergo, since winning while vastly outnumbered is extremely cool, it happens all the time.