It is so hard to predict these things because A) We don't know when Novak will start slipping, and B) we don't know who and when someone will emerge to challenge him.
My prediction of 18 Slams--6 more than he has now--is based upon several factors: that Novak has shown no signs of decline yet, that the tour is particularly weak right now and will continue to be weak through at least 2017, and that some decline is inevitable, although whether in 2017, 2018, or even 2019 we don't know.
Novak has three Slams left before turning 30. If he wins all three, he will be at 15, have won seven Slams in a row, and I think not only be well situated to pass Roger and finish with 18+, but have a real chance to finish as the nearly-undisputed GOAT, at least of the Open Era.
If he wins two of those three Slams, he will be at 14 and still have a solid chance of passing Roger, but maybe more like 50-50, and will probably finish in the 16-18 range, depending upon where he and the rest of the tour are at a year from now.
If he wins only one of those three Slams, he will be at 13 on his 30th birthday and will not pass Roger; I think he would finish more in the 14-16 range.
So the point is, those next three Slams are huge--they are his window of opportunity. It is not that when he turns 30, all of a sudden he will rapidly decline, but I do think how many Slams on his 30th birthday will give us a much better baseline than we have now.