I think it will be around 19.5 Mn. I think PLA and whatever the next Pokemon game is will make this game sell less in the long term.
Bonus question: As someone who played Pearl on the DS once and hated it, I actually liked Shining Pearl a lot. I have around 100 hours on my save file, 100% completed the Sinnoh Dex and I am around 100 short of completing the National Dex. Might go back and complete it eventually. I really enjoyed the game but if you really loved the originals and/or Platinum probably spend your money elsewhere.