If Nadal is injury free, I think his career will look something like this:
Currently 9 slams
Australian Open: 1 right now, 1 more. If Djokovic continues playing as he is playing, he will win a lot more of this title, especially of this title. So this isn't a major Nadal will win a lot of
Roland Garros: 5 now, 3-4 more. His body can't hold out forever. Regardless, having just 3 more breaks all titles at RG, and four more is a record he holds alone. But his body will break down some time, as Fed's has. But probably much more drastically around Fed's age due to grinding style. Dolgo will probably become his next threat here, but not enough to hinder him unless Rafa is injured
WC: 2 now, 2 more. I think this will be a title Fed wins again, but Nadal can win another two of them at least. Then again, his body won't hold up for eternity. But I also think that Dolgo and Raonic have big chances at this slam, especially Raonic.
USO: 1 now, 0 more. If Djokovic plays as well here as he did at AO, if Del Potro gets back to his best game, and if Harrison, Raonic, and Dolgo do fully get to where they are hyped up to be, they will have really good chances here. Definately the worst growth place for Rafa
So that means at most (from my antiNadal calculations) he can end up with 16 majors but not surpass Federer. That being said, I don't know how the up and coming players will end up growing into or when Nadal's injuries will become too dangerous for him, so that is an "at most and probably not" estimate