(Side note : to my knowledge this is the only source of info about the Max Party Size in BG3. I certainly wish this info would have been available in the official FAQ. Or, save that, that Larian would have seen the recurring comments about Max Party Size and made the effort to put the info out, improving the quality of the feedback and discussion by allowing participants to have an informed opinion. But, hey, we all know how much Larian is interested in fostering better discussion, receiving quality feedback, and overall communicating with players ...)
In the interview, Swen gave the following information.
Max Party Size will be 4.
The "engine" is not the limit (programming is more accurate, I suppose, but anyway). They purposefully made sure that it can handle more (at least 6).
Larian fully expects that there will be mods allowing 6-adventurers party.
Given that Swen said he expects mods to exist for this, I think it's reasonable to expect that 6-adventurer party will not be an in-game option, but only accessible through modding.
2) Why I disagree with Larian's stance.
To be very very clear, I am not talking about the choice of a Max Party Size of 4. I'll happily play with 4 party members.
Now ... given that it's easy to mod for 6, it's also very possible that I'll go with 6. It's not clear that I'll be doing many playthroughs of BG3, but it's increasingly clear that I'll be playing BG3 for the story and exploration (not the combat). And from the point of view of narrative content, bigger parties mean seeing more companions' stories (and, perhaps, more companion-to-companion interaction, however little there is currently).
Overall, the default value of Max Party Size in the full game is very near the bottom on my list of priorities for the game.
What I am talking about is their stance on letting player change Max Party Size : Larian has made sure to code the game in such a way that the Max Party Size variable can easily be switched to 5 or 6, but they won't make Max Party Size a parameter that players can adjust in-game, instead requiring that players mod the game if they want to play with 5 or 6 adventurers.
a) Bridled software, arbitrary usage restrictions.
A video game is a software. Once I've bought it, I'd like some freedom in how I choose to use it.
The software can handle parties of 6. But Larian is giving us a deliberately bridled version, and we have to hack it to use it to its full capability.
To me, this has the same bad smell as DRM.
I also think that this usage restriction is very arbitrary.
Some people might want to spend their gaming time creating characters, building crate towers, assembling music bands, making badger dinners, etc. These uses are possible and allowed.
Some people might want to experience the main story with 6 adventurers. This use is possible but not allowed.
b) Confidence, goodwill, inclusivity.
There could easily be an option in the Game Settings called Max Party Size, with initial value 4. You could increase it, up to the maximum size the engine can handle. If you increase it, the game could have a pop-up window saying "Hi there. The game is designed for parties of 4 adventurers. If you increase this number, the game will not react to that change. Notably, combat encounters will not scale accordingly, and will become less challenging. You could probably try to make them challenging again by using the difficulty settings. But that is not the experience we designed, and we recommend you keep Max Party Size at 4". The "Continue" button could read "I understand, but I'll increase this number anyway". The "Cancel" button could read "Oops. I'll stick to the default value". It would really cost Larian little work to implement that.
In Darkest Dungeons, the devs explained during EA that the corpse mechanics was an integral part of their vision. As many players opposed it, the devs added an option to de-activate it. They had a strong vision, but were confident enough to let players play differently.
In Celeste, you can activate a lower difficulty mode. The game tells you that this is not the intended experience. But maybe you just want to overcome that stage you've been retrying for an hour and then switch back to normal. Maybe you're mostly interested in seeing the game's story to its end, and perhaps you won't switch back. Your choice. The game is widely praised for its approach to difficulty.
To me, these developers are worthy of praise. They are confident in the gaming experience they've created, and don't mind if some players are seeking something a little different. They are willing to add options for those players. They are welcoming to people who have different levels of skill/ability or want a slightly different experience.
By contrast, by saying "no, really, we won't give you this option", Larian sounds insecure, lazy, and happy to exclude players seeking a slightly different experience.
c) Trust, respect, and communication.
One possible reason behind their stance is the following.
Larian thinks that, if players insist on using a 6-adventurer party despite the game's warning, and then have a lesser experience, they (some of them at least) are going to complain about the 6-adventurer gameplay being less good.
If the previous sentence (or some mild variation of it) is true (or somewhere close to true), then it would be very disappointing. And a bit disrespectful. The game is for mature audience only, who can be trusted to know how to read and comprehend written messages. So I certainly hope that the hypothetical argument above is not actually Larian's reason for making Max Party Size not adjustable in-game.
The best way for Larian to let players know the message that "yes, you can technically play with a party of 6 but, no, it is not advised", is to explicitly communicate this. Which can be done, for instance, via a pop-up message in-game when players increase Max Party Size above 4.
Meanwhile, saying nothing, letting players mod the game, and hoping that they'll get the hint and receive the implicit message, is a poor way to convey said message. (And, given how poorly Larian has communicated with players during the EA overall, I suppose that this is another facet of their poor communication philosophy.)