2021-03-19 : If Whirling Dervish dealt damage to an opponent who later left the game before that turn’s end step, its ability still gives it a +1/+1 counter.

2021-03-19 : Whirling Dervish’s triggered ability checks to see if it dealt damage during the turn to an opponent of its current controller. If it dealt damage to player A, then player A takes control of it, the ability doesn’t trigger.

2021-03-19 : Whirling Dervish’s triggered ability gives it at most one +1/+1 counter each turn, regardless of how many opponents were dealt damage beyond the first.