Yeah, I'd confirm that usually less than 5% of all of the sales of a game will amount in reviews. Usually it's closer to like 1-2%. It will differ from game to game though. And the reasons could be plenty. Remember that most people on Steam have a backlog of games, so don't be surprised if many of those who have bought one of your games, haven't actually played it yet at all.
Regardless, it's really pleasant to get nice, in-depth reviews of your game, so long as they're not toxic. A single review from a fan who genuinely enjoyed your game and talks passionately about it in a review or gives some interesting suggestions for you to consider in future projects is worth more than a dozen of negative reviews.
I'd also agree with what many said here that leaving a review is always a nice thing if you enjoyed the game. I always leave a review if I enjoyed a game, especially if it's an indie game that's not gotten enough attention yet. Even if your review is just a few sentences long, it's already a big help for the devs, and it is also a big help for the potential customers who are wondering if to buy the game or not.
