The monster manual on page 48 says…
Hit Points 80 (32d4)
but the next page says
Undead Nature. A demilich doesn’t require air, food, drink, or sleep. So great is the demilich’s will to survive that it always has the maximum number of hit points for its Hit Dice, instead of average hit points.
Old printings of the MM said its hit points were “80 (20d4)” this seems like it was correct assuming maximum HP, but the MM errata changed it to the current 80 (32d4). Although if you give them maximum number on their hit dice it sums up to 128 hit points.
So should Demilich Hit Points actually be 128?