> There's no limit to how many books the famous author can sell

There is, of course. The book is interesting within a particular range of cultures, and depends on good translations to be popular outside its native language area.

So, for a hyper-successful book, like "Harry Potter", the potentially interested audience is "merely" a few billion people, and "merely" a few hundred million are potential buyers of the book. 600M books sold worldwide, counting all the books in the series.

Selling 2x as much is highly unlikely, and 10x as much, completely unrealistic. The number of New Testament copies printed since 1450s is estimated to be below 7 billion.