In the description for the deck of many things magic item, there are two possible outcomes for how many cards are in the found deck:
Usually found in a box or pouch, this deck contains a number of cards made of ivory or vellum. Most (75 percent) of these decks have only thirteen cards, but the rest have twenty-two.
In the description for the magic item, there are 22 cards described. So if the deck is full, then every card on that description is in the deck. However, if the deck ends up only containing 13 cards, which cards are determined to be in this incomplete deck?