I think level 20 would be too much honestly,
As any experienced DM knows in 5e the 'fun' really starts when characters hit 14-17~ PC's become very very powerful by then especially casters, which makes balancing combat, or any sort of challenge really, much more complex.
It would be a huge accomplishment for Larian if they would be able to pull it off, to pay a respectable tribute to the complexity of D&D at those high levels,
Personally I would say 10 is a bit low, but 14 would be fine for me. But again, if they'd manage to pull of a full level 20 gameplay in detail with al mechanics in check, then this would easily be an RPG of mythical proportions.
OT: I also believe they envisioned level 10, but I;m not entirely sure if that is still relevant and accurate.
I feel that mechanical limitations in videogame adaptations tend to nerf some of the most heinous imbalances when it comes to caster advantage at later levels. Don't get me wrong, casters will still rule the day, but the gap usually closes a bit since the more creative applications tend to get ruled out.
E.G. Wish and True Polymorph will likely not work the same way as Table Top. Wish is simply too unlimited to capture that in current videogame programming. I can't imagine Larian keeping the permanence aspect of True Polymorph (although I believe AL limits that too), because it'll cause tons of problems in various aspects of the game (i.e. dialogue animations if you're in various forms, etc.,) so they'd rather just limit it.
We didn't see it as much in BG2 because the discrepancy in 2.5E was simply too big, but we did see a little bit of that in NWN 1/2, and Pathfinder Kingmaker (so 3E/3.5E/3.75E). Casters are definitely nowhere near as dominant (though still the best IMO) in those game compared to the respective table top versions. The gap in 5E is even smaller than those editions, so I think purely from a mechanical standpoint, it can work.
Whether going up to 20 actually makes sense and fits the story of BG3, that's another question.