The second set in the Time Spiral block, Planar Chaos features 165 cards with randomly-inserted premium cards.

Note:
Starting with Time Spiral, there has been a change in how premium cards are distributed. Previously, premium cards replaced a card of the same rarity, so a premium rare would replace a standard rare, a premium uncommon would replace a standard uncommon, and a premium common would replace a standard common. In Time Spiral and in future sets, all rare, uncommon, and common premium cards will replace a standard common instead. It is now possible for there to be two rares in one pack: the standard rare card plus a premium rare (which replaces a standard common).

For more information, go to the Planar Chaos Minisite