Each box of Set Boosters will be accompanied by the Forgotten Realms Buy-a-Box promo (while supplies last).
Set Boosters
With a focus on excitement and collecting, Set Boosters contain fourteen cards, twelve of which are playable Magic cards (in contrast with a Draft Booster, which comes with sixteen cards, fifteen of which are Magic cards).
Get more details in this article from Wizards.
Each Set Booster Box contains 30 Set Boosters
