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.
