The worst Demon can regen from even with their head along, can retact their limb if cut off and can grow new one if they lost those cut off limb. They can even grow new limb, body part if needed, or grow all kind of strange body part on their existing body, since they are demon, they are not completely abide to human form thus they can have 4 eyes, ten arms or whatever. Don't know if they can regen if their head got totally destroyed because no feats, but top-tier demon like Akaza or Kokushibo can regen even from head destruction and even body destruction, if there is a single part of body like an arm they possibly can regen from that and they resist regen negation, the reason they lose is because they willing to die. God tier Muzan can make limb out of his blood alone, so if there is still a pool of his blood he can regen his complete body
Yushiro regenerated from having his head smashed by Susamaru and Yahaba and I'm pretty sure that Tamayo stated that fights between demons are generally futile since they can't kill each other which wouldn't be the case if you could kill them by destroying their heads. There is also the implication of a fodder demon being able to recover from having their head completely crushed after having been reduced to a decapitated head.
