There are some people who might estimate the popularity of a book based on the number of copies sold. But, if you are a book lover, you might have noticed that in India, there is a different definition for a bestselling book. Unlike other countries, there are some other criteria in India for a book to become a bestseller in India.
The first thing implies the list of books that have been supplied by the venerable bookshop. However, in Amazon, the bestseller list is based on its own algorithms, and constantly checks for actual book sales and also combine it with the historical data analysis, to come up with a bestseller chart. If you look solely at the numbers, the accurate data will come from Nielsen-Bookscan which covers online sellers like Amazon, Flipkart, and also physical booksellers like Crossword.
The industry estimates that Bookscan covers about 60 to 70% of the trade book market. So the figure just gives us a basic idea of how many copies that got sold in a year. Amazon rankings are based on their own site sales. So, it can be an accurate measure of how well a book has done in India.
If we ask, how many copies of a book should get sold to qualify as a bestseller in India, it all depends on the goalposts. The target can 100,000 copies, and only 70,000 copies might get sold. Sometimes, a book with a target of 5000 can reach 30000. It is all about the perception of the author. Consider a number as the first barrier. If the can breach it, then it is on its way to becoming a best seller.