It's all of them for AT and EK and multiclasses. It has become especially fun with one of the recent patches when instead of one of your Spell DCs your own value of that stat is calculated in. For example my Astarion is a Rogue/Bard with a Spell DC of 17 (8 Base + 5 Charisma + 4 Prof) and 15 Spell Attack. Spells with a Dex Save display his Spell DC as 16 (8 Base + 4 Dex + 4 Prof), those with a Wis Save display 14 (8 Base + 2 Wis + 4 Prof). It's all calculated correctly though, it's just the display that is off.
