especially the Taj Mahal (which is the set with the most pieces in a set to date: with 5922 pieces).Any idea how many of them are unique pieces?
The short answer is: None of them.
That's the beauty of LEGO. No set has unique pieces, unless you count the detailed painted pieces. If a set ever comes with a "new" piece, you can count on that "new" piece be used in other sets released within that year or later.