From the publisher:

The Dominaria United Collector Booster Box contains 12 Dominaria United Collector Boosters and 1 Traditional Foil Box Topper card.

Each Collector Booster contains 15 Magic: The Gathering cards and 1 Traditional Foil double-sided token, with a combination of 5–6 cards of rarity Rare or higher and 4–5 Uncommon, 4 Common, and 1 Land cards, with a total of 10–13 foil cards. Foil Borderless Mythic Rare Planeswalker in 2% of boosters. A Lost Legends card (from the 1994 set, Legends) replaces a Common in 3% of boosters.

Contents:

• 12 Dominaria United MTG Collector Boosters + 1 foil box topper card
• 15 Magic cards + 1 foil token in each booster
• Lost Legends—card from the 1994 set, Legends, in 3% of Collector Boosters
• 10–13 foil cards in every pack—including at least 2 foil Legendary Creatures
• 5–6 cards of rarity Rare or higher in every pack