It's really tough to tell, because he's relatively young and, for now, healthy, but, it NEVER is as simple as extrapolating future results based on his most recent great results. There are always unexpected hurdles, weird losses, etc. When Fed was at his most dominant, there looked like there would be absolutely no slow down, people were predicting he would easily end up at 20+ Slams. But, tennis career arcs shift quickly.
That said, Rafa will obviously win more. At a minimum, and I know many people, will think I'm crazy for setting such a low limit, he'll win only 3 more Slams. I personally predict 5-7. And, like Fed, if we simply base it on his prime form and his current results, he could win 8+ more, though I wouldn't expect it.
The Rafa era is here. This may sound stupid, but his greatness completely sneaked up on me. Once he got to 6 Slams, he was a "great" like Edberg or Becker. But, Fed was far ahead, guys could beat Rafa, on occasion, by hitting through him (Tsonga, Del Potro), and it really looked like he would be unable to win the USO, there were real questions about his body and his injuries (which I'm sure will resurface sometime in the future to some degree), he couldn't defend his Wimbledon title.
But, he got fit, made a few alterations to his game, and suddenly after a period of 4 months is at 9!! Slams, with a career Slam. Whether he ever is considered the GOAT by a majority of people is yet to be determined, but just consider where he's at. It's pretty amazing. 9 Slams. If his career ended today, he'd arguably be Top 5 of all time.