However, the answer is 15. Each pack contains 10 commons, 3 Uncommons, 1 Rare, and 1 Land.
Extra-rare replacements take place as follows:
A Mythic rare takes the place of the rare.
A Foil card of any rarity takes the place of a Common.
A "Hidden Treasure" repackaged older card takes the place of the land.
Edit: Whoops, forgot the Token/Tips & Tricks card, as it is not technically a playable card. But every pack will contain, in addition to the 15 cards mentioned above, either a Token Card that represents one of the tokens that a card in the set can produce, or a Tips & Tricks card that details how a new mechanic works. The backside of these cards contains an advertisement rather than a magic card back.