Thoughts on Creativity In reality, by the way, speaking as a musician who has been on lots of albums, it's not at all as simple as us thinking, "If the album was sold, that's a person who likes us/who finds the music pleasing," so that we're basically thinking "We sold 500k copies of this. That's 500k people who like it."
Because we know that for example, some people could have bought it on a whim, or because they heard of us and they're taking a chance on it, but then it turns out that they don't like it.
Or, for example, someone who likes it bought a copy to try to interest someone else in it. But maybe the gift-receiver turns out to not like it.
Etc--although this sort of thing was more of a factor prior to the last 10-15 years. The Internet and streaming have changed things a lot. The number of people who buy albums on a whim without ever sampling any of the music is pretty negligible at this point.
At any rate, if we're selling 500k copies of a relase, we can be pretty sure that some of the people who bought it liked it, but we don't know just how many, just what percentage, we don't know just how much they like it aside from what people might say in reviews we might see, how they feel about it compared to other albums we released (ditto the last parenthetical), whether they'd rather we did more similar music or not, etc.
To a large extent it's always a shot in the dark. You know that there's some correlation to the gigs you're doing, the media mentions and airplay you're getting, the advertising you're doing, stylistic development you're undergoing, the amount of appreciation you're getting from fans, etc. and sales, but it's very difficult to ever directly attribute sales shifts to particular things, or to common opinions of the music--unless, for the latter, there's a major consensus about something considered monumentally good or horrible, so that everyone is talking about it. And at any rate, if you have the degree of fame required to know the latter, you can keep sales/a career going pretty indefinitely no matter what you're releasing, because you're popular enough, and had at least past work considered significant enough, that you can just keep riding the coattails of that.