Sifter Wurm

• WOTC Official Reminder Textofficial rules
  Scry — Look at the top N cards of your library, then put any number of them on the bottom of your library and the rest on top in any order.

• WWizards of the Coast2020-11-10
  Official Ruling
  For cards in your library with in their mana costs, X is considered to be 0.
• WWizards of the Coast2020-11-10
  Official Ruling
  Once Sifter Wurm's triggered ability begins to resolve, no player may take other actions until it's done. Notably, opponents can't try to change your library after you scry but before you reveal the top card of your library.

Rulings via Scryfall