The answer to this question will vary greatly, as best-seller sales depend on various factors such as the genre, author's clout, publicity, competitiveness of the market, and more.
That said, there is no definitive answer, as a book can be counted as a best-seller if it sells as few as 1000-1500 copies, or even more than a million copies.
It is also important to note that best-sellers are often determined by the specificity of criteria used by each organization to define it. For example, USA Today and The New York Times have different best-seller lists, based on the number of copies each book sells over a certain period of time.
Mostly, if an author's book reaches the New York Times' best-seller list, they have sold between 10,000-20,000 copies in the specified time period which would signify a best-selling book.
Overall, it is difficult to determine the exact number of books that best-sellers sell in a year, as that is largely dependent upon the specific criteria used to define a best-seller and other factors such as the genre and author's clout.
To learn more about best-sellers and their respective criteria, you can read more about it here:
• What Does It Mean to Be a Best Selling Author? (https://elasticwords.com/what-is-a-best-selling-author/)
• What is the Criteria for a Book Being a Best-Seller? (https://prowritingaid.com/art/2142/what-are-the-criteria-for-a-book-being-a-best-seller.aspx)
