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.)