Pact of the Serpent

• WWizards of the Coast2021-02-05
  Official Ruling
  The value of X is based on the number of creatures the player controls of the chosen type as Pact of the Serpent resolves.
• WWizards of the Coast2021-02-05
  Official Ruling
  You choose the target player as you cast Pact of the Serpent, but you don't choose the creature type until the spell resolves.
• WWizards of the Coast2021-02-05
  Official Ruling
  You must choose an existing creature type, such as Squirrel or Knight. You can't choose card types (e.g., artifact) or supertypes (e.g., snow).

Rulings via Scryfall