What APL should a LARGE party of 12 sufficiently geared/magic-itemed PCs be for a Deadly Encounter vs an Ancient Red Dragon in her Lair? APL 11? That's a tricky thing to balance. Would suggest beefing the dragon up with extra minions – very hard for a monster to compete with that many characters. Consider treating the group as three groups of four, building deadly for each, then combine

— Mike Mearls (@mikemearls) March 29, 2018