Two. There are two events of the Wurm damaging an opponent, even if they are simultaneous, so the Wurm triggers twice.
Private Mod Note
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I'm a former judge (lapsed), who keeps up to date on rules and policy. Keep in mind that judges' answers aren't necessarily more valid than those of people who aren't judges; what matters is we can quote the rules to back up our answers. When in doubt, ask for such quotes.
Charnelhoard Wurm and Chandra's Ingition Interaction.