No, its maximum damage is, in fact, twenty. Rather than counting the number of non-undead cards, I find it's easier to calculate if you just count the undead and subtract. 20-4N where N is the number of undead in each player's hand. If you played it at a time where it only did 16 damage, then your opponent must have had one unnoticed undead in his or her hand. Of course, since it affects you as well, you want to make sure you have five or more undead cards in your hand at the time of use to take no damage yourself.