Jdiver is correct that the 25k international one class upgrade includes up to 3 sectors. Should your itinerary include more than 3 sectors one way then you get to choose which 3 sectors you want to have upgraded. Also, if you purchase your flights separately and merge the records you can also apply the international upgrade to any sectors you wish.

I did this regularly when I was booking MAD-MIA-LAX-STL roundtrips. The LAX-STL portion was a separate reservation merged with the MAD-LAX routing. No problem applying a single 25k upgrade to the entire itinerary.