In a multiplayer game, if a source deals damage to multiple opponents at the same time, the last ability of Chandra's Spitfire will trigger as many times as there are opponents who were dealt damage.

The last ability of Chandra's Spitfire triggers just once for each event in which an opponent is dealt noncombat damage, regardless of how much damage that player is dealt.

Combat damage is the damage that's dealt automatically by attacking and blocking creatures. Any other damage is noncombat damage, even if it's dealt during a combat phase by an attacking or blocking creature.