And then you start DMing, even as a beginner who has no idea what he's doing you get 6-8 players at the table, that party size is unsupported by the balancing tools given by WotC so you wrack your brain trying to come up with challenging encounters for the whole group, burn out, stop DMing, and the imbalance worsens even further.
Just put your foot down and refuse to add more than 5 players until you're experienced enough. (*sigh* I wish I had followed my own advice.) Sure, CR started with 7 regular players and sometimes they have even bigger parties with guest players... but that's Matt Mercer, he knows what he's doing, and the players are also good enough to avoid making it a nightmare for him and each other. You as a noob DM will have your hands full with herding 4-5 overly excitable cats and managing the rest of the game.
This was my motivation behind allowing more players in the first place. My previous group basically fell apart because I had 3 players and when one of them canceled last minute - which was basically all sessions - there weren't enough players for a session. When one of those players dropped out permanently, I went online to look for more players, and I figured: "Hey, if I have 6 players with the same flake ratio then I'll have a 4-person party most of the time. Let's do this!"
But then the fuckers started showing up consistently for every session!
And even though I'm a noob (well, not so much since I've been DMing for a year now, 7 months of this for this large weekly group... but I still have no idea what I'm doing) I still have to make some new applicants take a number. Which just underlines how much of a DM/player imbalance there is.
(I'm planning to split the group based on player experience level into two biweekly campaigns of 5 players, that way I might be able to allow a couple of newer players to join and still preserve what was left of my sanity.)
I wonder if there would be a way to consistently split a single character across two or more people. Maybe that character has a fantasy version of dissociative identity disorder. Sometimes they go multiple sessions under control of one player or another, and sometimes there are multiple players in the driver's seat in a single session, either taking turns or controlling different aspects of that character, a la Everyone is John.