Time Spiral features 301 cards, plus 121 special "timeshifted" reprint cards from Magic's past, with randomly-inserted premium cards. All cards, including the timeshifted cards, are tournament legal. The set includes 121 commons, 80 uncommons, and 80 rares, plus the 121 timeshifted cards with their own rarity.
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).
Look for the following key cards:
Flagstones of Trokair
Teferi, Mage of Zhalfir