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.