Merfolk Windrobber — ruling 1

An ability that offers a bonus if an opponent has eight or more cards in their graveyard won’t provide additional benefits if more than one opponent has eight or more cards in their graveyard. There are also no additional benefits no matter how many cards are in an opponent’s graveyard, as long as there are at least eight.

Merfolk Windrobber — ruling 2

In a multiplayer game, once an opponent leaves the game, they won’t be considered for such an effect, no matter how many cards they had in their graveyard before leaving the game.

Merfolk Windrobber : règles, fonctionnement et interactions (MTG)

Merfolk Windrobber is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Flying Whenever this creature deals combat damage to a player, that player mills a card. (They put the top card of their library into their graveyard.) Sacrifice this creature: Draw a card. Activate only if an opponent has eight or more cards in their graveyard.

How it works : Merfolk Windrobber

Original English text — Wizards of the Coast / Scryfall

#1September 25, 2020· Wizards of the Coast
An ability that offers a bonus if an opponent has eight or more cards in their graveyard won’t provide additional benefits if more than one opponent has eight or more cards in their graveyard. There are also no additional benefits no matter how many cards are in an opponent’s graveyard, as long as there are at least eight.
No other card is named in this ruling.
#2September 25, 2020· Wizards of the Coast
In a multiplayer game, once an opponent leaves the game, they won’t be considered for such an effect, no matter how many cards they had in their graveyard before leaving the game.
No other card is named in this ruling.