It's hard to put much stock in his 2023 stats. He barely did anything at Barcelona, Rome, or especially Roland Garros. On the other hand, he's a young, improving player so I wouldn't expect his stats to regress back to his career average, either, and we've seen those improved stats pay off indoors, so it's possible the same happens on clay.
But I also think it'll be his weakest surface. He's just not as good a mover as Nadal, Djokovic, or Alcaraz. He's so strong in the fundamentals of the game that I think he can translate his play, Agassi-style, to any surface or condition, but I don't think he'll ever be a dominant clay courter. Just one among the top pack – which is still a great place to be. Plus he doesn't seem interested in playing lots of small clay court tournaments, especially outside the primary European clay swing, so that'll limit how many titles he can rack up.
For perspective, Djokovic is sitting on 19 titles after almost 20 years of playing on the tour. He had Nadal standing in his way for basically that entire time, which Sinner won't, but for Sinner to better Novak's numbers and go 20+ he'd still have to be close to Novak-caliber on the surface, and with his less-than-all-time-great speed and generally higher-risk play style, I don't know if I see that happening.