Before falling in the "power level wizard masturbator" category, I'd like to say that I never played such a game in solo (or even without a companion slot empty). The reason why I want a 6 party base game is because I love more customization and more management.
I was talking about it to introduce the question about balance and difficulty. I absolutely don't want the game to be designed for a solo character if that means it will be over easy with 4 or more characters. That's absolutely not what I wrote.
No one had an experience with a D&D campaign ? I.E Descent into Arvernus is designed for 4 to 6 players. What are the difference when you DM it for 4 or 6 friends ? Is that only a question of one or two more monsters in encounters ? Something else ?
Because whatever are our experience with video games, the rules comes from D&D so that could give us (me?) a better overview of what should really be re-balanced with more (or less) companions.