Unknown. Initially it was to be up to level 10 due to work required to implement spells. In, I think, RPS interview some months ago it was said the level cap will be most likely slightly higher then that, but there is no word of how high it will be.
If they stay true to 5th edition it should be level 20.
Single campaign doesn't need to cover all possible levels. As a matter of fact, more focused progression of BG1&2 I think worked better then single games spanning from zero to hero (Icewind Dales, PoE1&2, Kingmaker)