He will win 20. This era is so strong that the AO organisers will think that you cannot fit all these epic matches in two weeks and every year they will just give it to Djokovic for winning a final that lasted 6 hours.
Only two players have ever won a slam 7 times - Nadal at RG and Sampras at Wimbledon. These guys are considered by many to be the best clay and grass court players in history respectively.
Point is, 7 is a very impressive count. Is Novak the greatest hard court player of all time? Because I think that's what you'd have to be to win the Australian or US Open that many times. There's a lot more competition on that surface. Agassi was incredibly good at the Aussie open and he only got 4. I can see Novak getting 4, but not 7. He's already 25. There's simply not enough time.