Chancellor of the Annex

• WWizards of the Coast2011-06-01
  Official Ruling
  If you reveal more than one Chancellor of the Annex from your opening hand, each will trigger when an opponent casts their first spell of the game. That player will have to pay that many times or the spell will be countered.
• WWizards of the Coast2011-06-01
  Official Ruling
  In a multiplayer game, if you reveal Chancellor of the Annex from your opening hand, it will trigger each time an opponent casts their first spell of the game.
• WWizards of the Coast2021-03-19
  Official Ruling
  A player’s “opening hand” is the hand of cards the player has after all players have taken mulligans. If players have any cards in hand that allow actions to be taken with them from a player’s opening hand, the starting player takes all such actions first in any order, followed by each other player in turn order. Then the first turn begins.

Rulings via Scryfall