[Sheap]

There's no theoretical maximum regeneration on the Eater of the Dead. It all depends on how many corpses you can feed him. By sending him regular gifts of troops to eat, you can make an Eater very, very large.

I don't know exactly the maximum regeneration you can get without him becoming Unfettered. Of course, if you got him to that point, you'd only get to use him once before he would unfetter himself.