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Khans of Tarkir Booster box contains 36 packs of 15 cards each. Release date is September 26th. Khans of tarkir is the 65th Magic the gathering expansion and is a large set. The set was previewed at San Diego Comic-Con on Saturday, July 26, 2014. Mark Rosewater (Magic head designer) moderated a panel featuring Mark Purvis (Magic director of global brand marketing), Doug Beyer (Magic senior creative designer), and Gavin Verhey (Magic designer).
Khans of Tarkir is part of the Khans of Tarkir block, which will be Large-Small-Large with a unique draft structure (the small set is drafted with both large sets, but the large sets are never drafted together) and a time traveling element. The draft structure is the first thing from which Khans of Tarkir set was built 
Khans of Tarkir contains 269 cards (20 basic lands , 101 commons, 80 uncommons, 53 rares, 15 mythic rares), and includes randomly inserted premium versions of all cards. Like Scourge before, the set is Dragon themed. The expansion symbol depicts crossed swords on a shield, similar to the symbol of Legions.
Because of post-release changes in the block structure, Khans of Tarkir and all future sets will only be played in Standard for 18 months. 
Tarkir is a world embattled with ambitious warlords and powerful clans that wage eternal war for supremacy of their plane, a conflict that spans over a thousand years. Tarkir is also the home plane of Sarkhan Vol. It was previously inhabited by dragons, all of whom were killed before Sarkhan's period of servitude.  The clans each worship one aspect of the extinct dragons. 
The clans are represented through enemy wedge colors and aspects as follows:  
Notice that while the shards of Alara were centered around the color that had both allies, the wedges of Tarkir are not built around the color that has the two enemies. When the design team built the clans to match the flavor of the dragon attributes and the creative take on the clans, they found that they centered not on the enemy color but one of the ally colors. Everything worked so naturally, though, they felt it was best to leave it be and not force it another way.  By shifting the mana costs such that the enemy pairs were sitting next to each other in mana costs, it became a little easier to recognize that each wedge was made up of two enemy-color combinations. 
A preview of the set's artwork was aired during Pro Tour Journey into Nyx, following the quarterfinal rounds.  In it Mark Rosewater (Magic head designer) announced the return of something "that people have been asking [ Wizards ] to do for quite a while [...] after a long absence from magic" as well as the introduction of something that has been requested for a long time but had never been done before. These elements turned out to be morph and wedges, respectively, when the set was previewed at San Diego Comic-Con on Saturday, July 26, 2014. Mark Rosewater moderated a panel featuring Mark Purvis (Magic director of global brand marketing), Doug Beyer (Magic senior creative designer), and Gavin Verhey (Magic designer).   .
Khans of Tarkir will be sold in 16-card boosters (one card being a marketing card), five intro packs, one event deck and a fat pack.  As of Khans of Tarkir, the face cards in Intro Packs will not just be premium rare cards, but alternate art premium rare cards that won't be found in booster packs.