Dwarven Catapult deals X damage divided evenly, rounded down, among all creatures target opponent controls.
10/4/2004 Count the number of creatures on resolution of the spell and divide X by that number at that time. If the number of creatures changes before resolution, the damage done may be different than what you expected.
9/16/2007 For example, if X is 5 and the targeted opponent controls three creatures, Dwarven Catapult deals 1 damage to each of those creatures.
2/1/2014 If the targeted opponent controls more than X creatures, no damage will be dealt to any of them.