Titles with the most copies sold each week get onto the New York Times Bestsellers List, according to Writer's Digest. There are multiple bestselling lists, but all are based on the same principle of most books sold.
A bestselling book usually gets onto bestselling lists by outpacing other books in its genre for sales during a particular week. Different bestselling book lists draw from different sources to determine the order of their lists, such as the New York Times using only sales from retailers and Nielson Bookscan. Each bestselling list also creates its cut off period for weekly or monthly book sales. The New York Times follows a Saturday to Saturday schedule for weekly book sales. Amazon seems to update automatically on a daily basis.
The number of sales required to get onto a bestseller's list also varies by genre and time of year. Some genres don't sell a lot of books, so a relatively modest number of sales can get titles on to the bestselling list. Other genres have very high sales or a broad range of titles, so competition is fierce. Authors also set up book signings at bookstores each day of the week their books are first released to help boost sales and public interest.