From the Publisher:
Each Set Booster contains 12 Magic cards, 1 Art Card, and 1 token, ad card, Helper card, or special card from Magic's history (a card from "The List").
Each pack contains a combination of 1–4 cards of rarity Rare or higher, 2–7 Uncommons, 3–8 Commons, and 1 Land card. One non–Art Card and non–Land card of any rarity is traditional foil.
A foil-stamped Signature Art Card replaces the Art Card in 5% of Set Boosters. A card from The List replaces the token or ad card in 25% of Set Boosters.