You can activate Sheoldred’s last ability if any opponent has eight or more cards in their graveyard, not necessarily all of them.

Once you’ve activated Sheoldred’s last ability, it doesn’t matter what happens to the number of cards in opponents’ graveyards. The ability will resolve even if no opponent has eight or more cards in their graveyard by that time.

To resolve Sheoldred’s enters-the-battlefield ability in a multiplayer game, the next opponent in turn order (or, if it’s an opponent’s turn, the opponent whose turn it is) chooses a creature they control to sacrifice. Then each other opponent in turn order does the same. Then those creatures are sacrificed at the same time.

To resolve chapter II of The True Scriptures in a multiplayer game, the next opponent in turn order (or, if it’s an opponent’s turn, the opponent whose turn it is) chooses three cards in hand. Any opponent who has fewer than three cards in hand chooses all cards they have in hand. They set the chosen cards aside without revealing them. Then each other opponent in turn order does the same. Next, all chosen cards are revealed and discarded at the same time. Then, each opponent mills three cards.