The value of X will change as the number of creature cards in the enchanted creature’s controller’s graveyard changes. Any time state-based actions are performed, if that creature’s toughness is 0 or less, or damage marked on it is equal to or greater than its toughness, the creature will be put into its owner’s graveyard.
Enchant creature
Enchanted creature gets -X/-X, where X is the number of creature cards in its controller's graveyard.
