Oracle Text

Destroy all nonblack creatures. Hellfire deals X plus 3 damage to you, where X is the number of creatures that died this way.

Card Rulings

10/1/2009 Hellfire checks the number of creatures put into graveyards as the result of its effect, not the number of creatures destroyed as a result of its effect. For example, a replacement effect that says “If a creature would be put into a graveyard from the battlefield, exile it instead” won’t affect whether a creature is destroyed or not, but will affect whether it’s put into a graveyard or not.
10/1/2009 A creature that regenerates instead of being destroyed doesn’t leave the battlefield, so it’s not put into a graveyard.
10/1/2009 A token creature that’s destroyed is put into its owner’s graveyard, and will be counted by Hellfire’s effect. (Immediately after Hellfire finishes resolving, the token will cease to exist.)