More than likely for a double IPA. Or make a yeast starter. Depending on what you OG is and how old the smack packs are you could probably just pitch two packs. Happy Brewing!
2 would be better than 1 but depending on your gravity and the age of the yeast packs, you might need 3 or 4 packs. That gets expensive really quickly so a starter would be your best bet. Use Mr. Malty Yeast Calculator to figure out how much yeast you need for your batch.
With that gravity you would need 3 fresh packs of yeast or a 3 liter starter if you are shaking intermittently to aerate or a 2 liter starter if you have a stir plate.