I'm always interested in how books become best sellers. Some have been crap. Not just poorly written, because that's just style that I don't like, but poorly th

I'm always interested in how books become best sellers. Some have been crap. Not just poorly written, because that's just style that I don't like, but poorly thought out. How many sales does it take to make money off of books? Is it way more attainable than I assume?