Passing Roger means three more Slams...and that assumes that Roger doesn't win another.
I know we saw what we saw at Roland Garros, but to win three more is a tall order. First and foremost, he has to stay healthy. Not just healthy enough to play, but healthy enough to win. Secondly, the young guys are getting better. Still a far cry from Rafa on clay, but the gap is narrowing - and tennis, like all sports, is a game of margins. Maybe a 25 year old Thiem can't beat a 33 year old Rafa, but what about a 28 year old Thiem and a 36 year old Rafa? Or what about a 21 or 22 year old Tsitsipas? The point being, Rafa won't get any better...he'll only lose a bit here and there, while the young guys will continue to improve. Couple that with, say, an achy knee, and it is only a matter of time before we see Rafa lose at RG.
Now there are three other Slams. But while Rafa is still an elite performer off clay, he's a mortal elite player. He might win another USO or AO, but the competition is thick and getting thicker.
As far as Rafa's actual chances, I'd say he has about an 80% of winning another Slam and then it reduces by about 20% per Slam. Like so:
My guess for how many more Slams Rafa wins:
0 more (18): 20%
1+ more (19+): 80%
2+ more (20+): 60%
3+ more (21+: 40%
4+ more (22+): 20%
5+ more (23+): <10%
6+ more (24+): <5%
I also wouldn't be surprised of a scenario were Roger, Rafa, and Novak all finish with the same number...either 20 or 21. Either way, I do think they'll all end up within 2 Slams of each other, so somewhere in the 18-22 range.