If an opponent has eight or more cards in their graveyard, Anticognition can still target only a creature or planeswalker spell.

Anticognition will counter the spell and let you scry 2 if any opponent has eight or more cards in their graveyard, not just that spell’s controller.

Anticognition can target a spell that can’t be countered. You’ll still scry 2 if an opponent has eight or more cards in their graveyard.

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.

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.