Sorcery X is the number of artifact and/or creature cards in an opponent's graveyard as you cast Spoils of War. Distribute X +1/+1 counters among any number of target creatures.1
4/1/2008: A "creature card" is any card with the type Creature, even if it has other types such as Artifact, Enchantment, or Land. Older cards of type Summon are also Creature cards.
8/1/2008: X is set as you cast the spell to be exactly the number of appropriate cards in an opponent's graveyard. You can't choose a higher or lower value for X.