Party size

[Hilary]

So... with the whole power in numbers action economy... why would adventurer's limit themselves to groups of 5 or 6? Why not run around in gangs of 100 or more? That would crush.

[Nidgit]

Not enough loot to pay for it. Also, sometimes stealth is important and it's difficult to do that with so many people.

[Kornaki]

This is true in the real world - force strength grows like, quadratically with the number of people or something, but each seal team 6 platoon is only 16 people.

[Naanomi]

Where do you find 100 people who can all meet on the same night of the week?!?

[PhoenixPhyre]

Where do you find 100 people who can all meet on the same night of the week?!?

This.

And coordination issues grow exponentially (figurative, not literal) with team size. There's good research showing that 4-6 direct reports is the sweet spot for managers--anything more and you end up stepping on each others toes.

And it's really hard to fit in a dungeon with 100 people. Heck, I find I have to make my dungeon rooms unrealistically large to fit 4 PCs and enough enemies to threaten them.

[KorvinStarmast]

So... with the whole power in numbers action economy... why would adventurer's limit themselves to groups of 5 or 6? Why not run around in gangs of 100 or more? That would crush.

To answer this question, you need to spend the next six months of your life as the DM. That will answer it in a way that you'll never forget.

[MaxWilson]

So... with the whole power in numbers action economy... why would adventurer's limit themselves to groups of 5 or 6? Why not run around in gangs of 100 or more? That would crush.

Because the PHB initiative system already requires players to spend 80%+ of their time in combat sitting around doing nothing. At 1% they'll go home.

Groups of ten or more adventurers work in D&D or AD&D but not in 5E.

If you're looking for an in-world rationale I'll just ask you, "How dangerous is the campaign world?" If it's not dangerous they may have no real reason. If it's very dangerous (like Spelljammer and Planescape) then groups/factions of high-level individuals may be an important part of the campaign world--but they tend not to be involved in large numbers in onscreen fights because again, 5E's default initiative system doesn't scale, and most 5E DMs use that initiative system.

For my part I prefer to kill off most high-level friendlies before the campaign starts, in order to create the chaos that makes the PCs necessary/important. Catastrophe leads to good gaming experiences fixing it.

[pragma]

I think small groups of adventurers make sense from a ruler's perspective. Projecting military force into hostile, unmapped territory is expensive, often politically unpopular, and sometimes existentially unsafe. Why not try paying a few wandering criminals adventurers a fraction of the cost of sending an army to get the same results? There's always more people looking for gold if the first expedition doesn't work out.

[Duff]

There's a couple of things...
An adventuring party are generally close - they trust each other with their lives. If you're rolling in with a small army, you don't really know everyone, so there's more risk of a knife in the back.

Limited space - in a lot of fights, there's only room for about a dozen people in the fight before they're getting in each other's way. 100 people lined up in a 10' wide corridor is not more effective than maybe 10 or so at most (though if you're expecting to run at a death rate, having spares is good)

Also logistics - a band of 6 can stay in inns, walk through towns without scaring the locals, buy horses in one town and sell them in another, find cabins on a ship going in the right direction and join with a caravan to cross the desert.
100 people need people ranging out in front and around to make sure there's enough food and water along the way. They don't get let into the town for fear they'll take over. If they need horses, they need a large town or small city to be able to buy all they need, and when they get to the coast, they are going to flood the market before they have to buy or charter a large ship to get them to where they want to go.

[Tanarii]

Stealth, fireballs, and dungeons being fairly constrained spaces.

Druids are specially fun. A single Moon Druid makes dungeon combat hard quick, since they're often using Large forms. Once Druids start conjuring beasts it gets claustrophobic real quick.

[Luccan]

On a meta level, that would be a nightmare to run and play in. In-game there are a lot of potential reasons, many of which probably have to do with the significant chances of death most career adventurers face.

[Darth Credence]

In my campaign world, there are absolutely groups that are that large. None that are players being focused on, but I have had players run across a group with something like 50 Dwarves and wagons full of equipment preparing to clean out a dungeon. The whole thing was just about adding life to the world, as well as dropping a few tips on the new players who haven't learned all sorts of fun exploits for different things. They saw them in action as the Dwarves loaded up and headed in, then the players went on their way and eventually picked up a few tricks that I had showed them work in my world, and extrapolated to come up with a few of their own.

Everyone else has covered why it doesn't actual work from a real life perspective.

[Connington]

Your average witch is not, by nature, a social animal as far as other witches are concerned. There's a conflict of dominant personalities. There's a group of ringleaders without a ring. There's the basic unwritten rule of witchcraft, which is 'Don't do what you will, do what I say.' The natural size of a coven is one. Witches only get together when they can't avoid it.

High level adventurers don't grow on trees. And those that do exist are just as likely to have obsessive personal quests or polarizing personality "quirks" as the Player Characters. They're even more likely to be tied to a specific organization or community that they can't abandon to go adventuring. The instability of an all-star team grows exponentially with size.

If we're just talking about fodder, most adventuring tasks are either too dangerous (at high levels) or not important enough (at low levels) to send in an army of henchmen.

Also logistics - a band of 6 can stay in inns, walk through towns without scaring the locals, buy horses in one town and sell them in another, find cabins on a ship going in the right direction and join with a caravan to cross the desert.

This is also super relevant. I've played in parties that numbered a dozen plus with NPC support, and the logistics of moving from town to town could be onerous. Triply so in modern world games, where you need a caravan of cars or have to book rows and rows of airplane seats.

The point about scaring the locals is also a major point if you're adventuring in town. Walking around with a mob of armed strangers is the next closest thing to yelling "WATCH OUT, WE'RE LOOKING FOR TROUBLE!!!" at the top of your lungs. Your enemies make themselves scarce, civilians clam up, and the local authorities take a vigorous interest in your travel plans.

[jojosskul]

Because gatherings of more than 5 are unlawful due to newly enacted plague regulations. Of course.

[Sigreid]

In game reason:

1. where are you going to find 100 people you would trust with your life.
2. need a lot of loot to make it worth it.
3. the local government is likely to be a lot more nervous about a private army in their territory than what amounts to a hand full of survivalists.

Out of game reason, once you get past 4-5 people it gets really hard to manage the game.

[KorvinStarmast]

Because gatherings of more than 5 are unlawful due to newly enacted plague regulations.

Beyond that, we must now all use reach weapons and ranged weapons due to social distancing.
Captain of the Town Watch The longsword and warhammer turn-in line is over there. You can purchase your new glaive for - wait, it was in the PHB for 20 GP, what's with this 28 GP price tag on a glaive? Price gouging?
Deputy assistant Watch Captain: Oh, and you monks with your fancy martial arts? Banned!

[Teaguethebean]

Because mobilizing an entire army is hard. Furthermore adventurers are extraordinary people, why split that loot with your swarm of lackies. Even more having 200 Soldiers doesn't help much when you fight a fire breathing dragon that will leave the nation with more widows than your left gold pieces.

[DwarfFighter]

So... with the whole power in numbers action economy... why would adventurer's limit themselves to groups of 5 or 6? Why not run around in gangs of 100 or more? That would crush.

COVID-19 guidelines have made me put that size of games on hold for a while now. Max 10 guests at private gatherings.

-DF

[jojosskul]

Beyond that, we must now all use reach weapons and ranged weapons due to social distancing.
Captain of the Town Watch The longsword and warhammer turn-in line is over there. You can purchase your new glaive for - wait, it was in the PHB for 20 GP, what's with this 28 GP price tag on a glaive? Price gouging?
Deputy assistant Watch Captain: Oh, and you monks with your fancy martial arts? Banned!

All unarmed strikes now deal poison damage!

[Malinthas]

The IRL reason is that games would slow to a crawl and be pure chaos. In-person, I can't imagine where I would fit 100 humans and be able to hear everyone when needed.

In-game, remember that adventurers are supposed to be extremely rare; exceptional individuals. The Conans and Merlins of their world. Even finding 4-5 like-minded and highly-powered individuals should be extraordinary.