Every booster pack includes exactly 16 cards. Out of these 16, one is guaranteed to be a leader card, and another is guaranteed to be a base (with either an Experience or Shield token on the other side). That leaves 14 cards left that are drawn from a pool of standard (non-leader, non-base, non-token) cards. Aside from the cards exclusive to starter decks, this game has four different rarities for its cards: Common, Uncommon, Rare, and Legendary. Each pack contains 9 Common cards, 3 Uncommon cards, and 1 Rare or Legendary card. Though you'll notice that only adds up to 13; add the leader and base card, and you’ll have 15. So what's the last card in the pack? It's a foil card, which is a special version of any card in the set that can show up in booster packs. This foil card can be any rarity, so you could get really lucky!
