Balancing combats is the easiest part of a level 20 campaign, especially if you want to do what Larian is doing and create a reactive campaign that tries to honor all options players have. Doing it at the table from session to session is far easier as you can adapt as DM and also you don't need to develop anything to even have the possibility to use something and instead just use the rules. If the first 12 levels where about 600 abilities and spells, I would assume the remaining 8 levels would probably add 200-300 more and probably with far wider reaching abilities than a 'heal wounds' or 'fireball' or a basic 'attack'.
