Skyrim's current sales figures are closing in on 50 million. Witcher 3 is well over 40 million. CP2077 closing in on 20 million. Even Obsidian's Grounded has sold over 10 million. Starfield will surely be in the same box as these games, sales-wise, though BG3 surely will not.
I am actually really interested to see how BG3 does sales wise. It's not exactly in a genre that is going to permit it to break any kind of crazy sales records (seems like you have to be mobile, child oriented, FPS or some big open world action title type thing) - but it is a pretty beloved franchise. Sven said sales were "insane" already and in October of 2020 it was reported that over a million copies had already sold.
I imagine it settles in somewhere between the 5 million and 10 million mark depending on how well received the actual release of the game is. I'm also curious what Larian would consider to be very successful given that the budget for BG3 has to be massive compared to D:OS2.
Yeah, for me, BG3 would need to sell 10 M to be considered a true breakout hit in its genre. I believe it is at around 3 M right now, and 5 M is something many other games in this genre have been able to hit, such as DA:O. DA:I is over 10 M.