in pathfinder wrath of the righteous.. game was designed with 6 party characters. there seems to be people who played with 4 too and even solo. it's doable. but i won't be punishing myself nor am i having the guts to play the game solo. i'm having a tough challenge even with 6. there are alot of options in regards to the difficulty. like more enemies, enemies with scaled stats and enemies that behave differently as well like.. marking your squishies as sworn enemies.
since pathfinder can implement it, i believe larian can too. the question is only whether if they want to. bg3 has a AAA budget for an RPG while pathfinder wrath came from a small indie studio with no huge budgets. while bg3 looks very appealing visually, the core of the game is still the combat. so i hope larian would give us the options for 6 and customized difficulty options for people who wants to play the way they like to play it.
The idea is that you should always start big and then go smaller. If you develop the system for a larger party, you can make it so that it is smaller. If you try to do the reverse, that's when you run into difficulties. It is always more challenging to try to implement a larger party size after you've already based everything on smaller.
just to add.. i kickstarted pillars of eternity 2.. there was a huge backlash from the community and there are certain obsidian defenders trying to silent and bash whoever that brings up the 6 party character discussion. see how pillars of eternity 2 reception now. it was a financial failure. of course the issues are not just the party characters alone but it's one of the biggest hightlight. even a tweet from "critical role" about that 5 party characters. there are only few new characters. 3 of the existing characters were returned from pillars1 even. moral of the story don't alienate your core fanbase.