Here's why:
* No official, up-to-date number exists: Sales figures for books are not publicly shared with that level of detail, especially for older titles.
* Sales figures vary: The number of copies sold depends on factors like:
* The book's format (hardcover, paperback, ebook, audiobook)
* The language it's translated into
* The year the book was sold
However, we can say this:
* It's a massive bestseller: "Harry Potter and the Half-Blood Prince" is one of the best-selling books of all time, and within the Harry Potter series, it's one of the most popular.
* Estimated sales: While an exact number is impossible, many sources estimate that the series as a whole has sold over 500 million copies worldwide. That means "Half-Blood Prince" alone likely sold tens of millions of copies.
If you are looking for a specific piece of information, please be more specific about the format, language, or time period.