Brik Wars invokes this trope for its Ninja Scum◊ card, resulting in a unit that gets lower rolls the more there are (and you must have at least three). The flavor text says it all.
Don't Rest Your Head has a Ninja madness power that lets you summon endless numbers of ninja mooks from every impossible hiding place. Their only ability is a reckless Zerg Rush. OR you call an elite Ninja, Colour-Coded for Your Convenience. Don't mess with him. He's badass.
Dungeons & Dragons Fourth Edition uses "minion" class enemies to invoke this trope in 4th Edition. They each have one hit point and are designed to fall in droves. Then there are elite and solo enemies, equal to two and five normal monsters respectively. If you see a group of 20 orcs, they are probably mostly minions, and one fireball will leave you with a target or two left; if you see two orcs, they are probably elite brutes, each of which has 194 hit points, a much nastier fight.
The 3rd edition of D&D shows a different form of this trope, in that the Encounter Level (difficulty) of a fight is calculated not by counting the enemies, but by adding 2 to the Encounter Level for each doubling of the number of enemies. Thusly, one gnoll is EL 1, but sixteen gnolls are only EL 9. By the time you get up to 32, it's not even worth raising the EL, as characters above 9th level have enough mass-effect spells to easily handle that many weak enemies.
In the original editions, fighters had the ability to attack a number of enemies in one round based on their general level, provided that the enemies were 1 HD or less each.
Even more so in the Battlesystem rules, where a unit of 10 or so mooks had what effectively amounted to 2 HP, and could be easily killed by a moderate level "Hero" (read: PC or leveled NPC) in a single round. A unit who succeeded in dealing a Hero a "wound", though, actually took off multiple HP ... 4, if I remember properly, though it's been a while.
One edition had constructs called Shardsoul Slayers, basically fragmented elementals which traveled in groups. Killing one united its shard of elemental soul with another one close by, making it stronger.
As a final note, this only applies in the gameplay side of Gameplay and Story Segregation, because any Game Master who isn't a complete dope will be fully aware that a hopeless and pointless battle for the players is not fun. Thus, dismissing the 20 orcs as "most likely minions" is a Metagame call which the characters have no way of making themselves.
Crimson Skies click-base version used a technique similar to D&D's "minion" system for its person-to-person combat branch: One hero could demolish large numbers of Mooks. In fact, like D&D, not only did the Mooks have but one hit-point, they were represented on the battle map not by a miniature (as heroes were), but by a cardboard counter. You see, they were "token Mooks"....
The Ninja Burger Employee's Handbook specifically recommends against this trope.
A particularly nasty version of this occurs in Runequest; due to the Critical Fumble rules, armies take the most damage from their own side. Thus, a larger army is actually less of a threat.
7th Sea has three types of enemies, mooks, henchmen, and villains, with increasing toughness.
Warhammer 5th edition got the nickname Herohammer because of this: individual heroes tended to be much stronger and potent than entire units of soldiers.
The game's rules have often tried to subvert this. Currently, the rules favor huge blocks of infantry, with the faction that can field the largest numbers, the Skaven, often regarded as a Gamebreaker due to the sheer numbers they can put out for their cost.
Warhammer 40,000's fluff shows space marines as near invincible in small numbers, but die in droves when it comes to large scale engagements.
Admitedly, in the 41st millenium EVERYTHING dies in droves in large battles. Also, a Space Marine force barely a hundred strong (a company) is fully capable of taking out a whole planet or even a small star system all on its own.
On the tabletop, each side has a finite number of 'points' to spend on models for their army. Mooks range from four to twenty points, commanders are at least a hundred.
Applied as a rule in the RPG, grouping many weaker enemies into "hordes." A horde effectively acts as a single unit, and its strength and "health" is effectively represented through its size. While individual enemies have health tracks that allow them to (theoretically) endure a number of attacks, any attack that can do damage to a unit in the horde will reduce its size, effectively allowing you to kill five to ten (or more) units in a single attack.
Certain weapons that don't work very well on individual units tend to be very effective against hordes, particularly flamer weapons and grenades. A lot of this has to do with hordes being unable to dodge, which is the primary defensive tactic against flamers and grenades.
This is pretty common place in tabletop strategy games, the more expensive a unit, the fewer of it you'll end up fielding.
The roleplaying game Pirates Vs. Ninjas has the universal law called the Kurosawa Corollary, by which members of a much larger group of combatants take penalties to become less powerful than a smaller group of adversaries.
Exalted models this by presenting any sufficiently large number of disposable enemies as 'Extras', with drastically low health and stats, that exist mostly as a minor obstacle to the players, window dressing for the antagonists, or stunt fodder.
Magic: The Gathering has both played this trope straight and subverted it. Not taking instants and sorceries into consideration, some creatures have either strong enough or have abilities that mean even if an opponent has a greater number of creatures, they can still be at a disadvantage. That said, there are also creatures that can give large hordes an advantage with abilities like battle cry (which boosts attacking creatures, and multiple battle cries stack), and some creature types, like slivers and elves, while pretty harmless individually, have abilities that make them devastating if there's a lot of them.
One dramatic example in the storyline are the dragon engines, mechanical dragons from Phyrexia. In The Brothers War, Mishra gets his hands on four dragon engines, which is enough to destroy an entire city. Much later during the Invasion Cycle, there's a scene in which the skyship Weatherlight takes on twenty dragon engines alone and totally destroys them.
Swashbucklers of the 7 Skies, somewhat similar to 7th Sea mentioned above, rates potential antagonists on a scale of minions, sidekicks, lieutenants, villains, and archvillains. Minions don't even have the dignity of being dangerous on their own, but rather must join together in minion squads to hope to hurt a player character. At their absolute most dangerous they're Glass Cannons... if they even survive long enough to get off a "shot."
A Pyramid article on the "Horde Ninja" (ninja as Mooks in GURPS Dungeon Fantasy) has a game mechanic for this; Horde Ninja have several levels of the Higher Purpose advantage, with the purpose being "Avenge my fellows!" The more fallen ninja there are to avenge, the more bonuses the ones that are still standing get.