A petabyte can be defined as either 1000 terabytes or 1024 terabytes. The same with the other measures. This leads to endless confusion, which began in the days when K meant either 1000 or 1024. For example, if you had $1K of cash in your left pocket and 1K of RAM cache in your right pocket you'd probably have $1000 plus 1024 bytes!
Rough calculations...
If you stored 12 petabytes in Apple's 500,000 square foot North Carolina data center, that's 1.1E15 bytes in 5E5 square feet, or 2.3E9 bytes/sq. ft. In other words they'd have only about 2GB per square foot. They'll need a lot more data to fill up that building.
So let's figure it out. The latest disk drives might hold 4TB in 490,000 cubic mm, or 0.0173 cubic feet, which means you can store 2.4E5 GB per cubic foot. Let's cut that down to 1E5 to leave room for enclosures and cabling. The data center appears to be a one-story building so let's assume the ceilings leave 10 feet of vertical usable space. That gives us 5E6 cubic feet x 1E5 GB per cubic foot, or 5E11 GB. So they have room for 476,837 petabytes!