Last night, I was doing some research about how well sequel books sell compared to originals. There's a surprising lack of information out there on that subject, but I did find this helpful article about book sales in general: https://electricliterature.com/everything-you-wanted-to-know-about-book-sales-but-were-afraid-to-ask/
It's a few years old, but it's the clearest explanation I've found so far about what writers can reasonably expect once their books hit (e)stores. In fact, there's a lot in this article that I didn't know! Check it out—I think you'll be glad you did.
(Thanks to Marc Fanelli-Isla for sharing their work on Unsplash.)