one of the actual worst things that D&D instilled on rpg groups is the idea that you need a party of four. which i think came from the whole "fighter, rogue, wizard, cleric" template. and then 4E further refined that into the "Defender, Striker, Controller, Leader" archetypes, where you pretty much NEEDED each of those roles in your party.
but I still see this so much in other indie games as well where people will go for groups of 4+ and its just kinda maddening to me because anytime I have been able to have a group where its just me and three other players, that shit will sing. Pretty much any PbtA or FitD game is amazing at those numbers. Everyone's got a bond with the person to their left and right, its really easy to keep track of and going around the table is really snappy?
I remember there was this really cool looking mass discord blades in the dark event happening where all these games where going to be happening in tandem, and i know there was some thought to like interconnectivity or something. but i'd been somewhat interested in actually playing until the gm i was talking with explained how they were happy to have a group of 7+ and just... nah. you can split that up into two groups! Look, I love Friends at the Table as much as the next nerd but there's a good fucking reason they don't try and run a bunch of sessions where everyone is in the same place.
it is also nice because like... people are waiting for their turns less, there is less chance of people getting bored and zoning out. if you ARE doing something with initiative then you have the person before you and the person after you to worry about on your team. It's a lot less potential havoc.
kind of related: D&D4e's guideline is one of each role the game needs to work smoothly, and tells you what the roles are and why they're recommended, something a lot of games are very averse to explaining or don't seem to have considered?
There's the D&D3e/PF1e opaque class systems, where there is an intended way to play but party composition is more something you learn from other people who've played the game a lot rather than from the game (probably why 4e actually explained it to you, to help groups of all new players)
There's games like most PbtA games where party composition doesn't really matter because it's more character/story focused, but as Mitch said they still recommend 4 players for... reasons?
The one that annoys me most is classless systems who do the whole "you can be anything!" but don't actually work very well unless someone's character is covering a few specific bases, and don't really explain that. Using Shadowrun as an example, you don't need a wizard but you do need someone with astral perception, you not only need a hacker but they need a way to connect directly to things which can be covered by themselves or another character, and despite how much the fiction talks about them you don't need a street samurai the team just need to be collectively good at combat.
Just like... explain your game mechanics, blease, if the mechanics expect a certain team dynamic then "4-6 players" doesn't really cover it, and if they don't then you're putting people off of running for smaller groups for no real reason?