There are 249 black-bordered cards in Rise of the Eldrazi. (15 Mythic Rare, 53 rare, 60 uncommon, 101 common, 20 basic lands).
The contents of a booster is 10 commons, 3 uncommons, 1 basic land, 1 tip/token card, and 1 rare card. A Mythic rare will appear in the rare slot in approximately 1 in every 8 booster packs.
This pack contains fifteen cards.