Five-Alarm Fire

• WWizards of the Coast2013-01-24
  Official Ruling
  Five-Alarm Fire’s first ability triggers once for each time a creature you control deals combat damage, even if it deals combat damage to more than one creature, planeswalker, or player. For example, an attacking creature with trample that deals combat damage to a blocking creature and the defending player will cause Five-Alarm Fire’s ability to trigger once. A creature with double strike, however, may cause Five-Alarm Fire’s ability to trigger twice during a single combat.

Rulings via Scryfall