hm, I can only assume humanoid identities, so maybe I've eaten a thrall. That'd leave me with a 12th level char at best and the basic greater doppelganger as characters, and maybe something else if I'd eat it.
The abilities of a GD aren't that great. They suck. They are as strong as the identities they can adopt.
To eat 12 20th level sorcerers, I'd need a lot of spells, I think, and to do that, I need to start with lower level wizards, and then work my way up, and the personality loss is random, so after a while I'd be losing 20th level sorcerers instead of 10th level wizards...
Maybe a max level of what I can consume?