Rule 603.2c

An ability triggers only once each time its trigger event occurs. However, it can trigger repeatedly if one event contains multiple occurrences. See also rule 509.4. Example: A permanent has an ability whose trigger condition reads, "Whenever a land is put into a graveyard from the battlefield, . . . ." If someone casts a spell that destroys all lands, the ability will trigger once for each land put into the graveyard during the spell’s resolution.

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