Extinction Event

• WWizards of the Coast2020-04-17
  Official Ruling
  The mana value of a token that isn't a copy of another object is 0. A token that is a copy of another object has the same mana cost as that object.
• WWizards of the Coast2020-04-17
  Official Ruling
  If a creature on the battlefield has in its mana cost, X is considered to be 0.
• WWizards of the Coast2020-04-17
  Official Ruling
  You choose odd or even while Extinction Event is resolving. Players can't take any actions between the time you make that choice and the time some creatures go extinct.

Rulings via Scryfall