Rule 383.1.b

If an ability triggers “the [Nth] time” something happens and that trigger condition is met multiple times simultaneously, the ability’s controller picks one of those instances to serve as the trigger condition. The ability triggers only once, due to the chosen condition. Example: Wraith of Echoes reads “The first time another friendly unit dies each turn, draw 1.” That ability hasn’t triggered yet this turn. Two other friendly units die simultaneously (say, due to combat damage). The Wraith’s controller chooses one of those deaths to trigger Wraith’s ability.