According to Storage Newsletter, Apple has purchased 12 petabytes (1 petabyte is 1024 terabytes, so over 12,000 terabytes) of storage from EMC's Isilon Systems, reported for use with iTunes to store music, movies, TV, and other media content online.
Obviously this will be hugely expensive -- you don't buy this level of storage from Best Buy, after all -- but storing huge amounts of data with both staging, local, and off-site backup doesn't come cheap. If Apple wants to offer something similar to Amazon's Cloud Locker service in the future, this much storage will certainly come in handy.
It's interesting Apple is choosing to go the Isilon route rather than use massive amounts of commodity drives the way Google does. Any data experts out there have a theory on that?
(And yes, if you're trying to figure out how many MKVs you could store on 12PB the answer very nearly is -- all of them.)