Rule 201.3

If an effect instructs a player to name a card, the player must choose the name of a card that exists in the Oracle card reference (see rule 108.1) and is legal in the format of the game the player is playing. (See rule 100.6.) If the player wants to name a split card, the player must name both halves of the split card. (See rule 708.) If the player wants to name a flip card's alternative name, the player may do so. (See rule 709.) If the player wants to name the back face of a double-faced card, the player may do so. (See rule 711.) A player may not choose the name of a token unless it's also the name of a card.

Find this number in the current Rules