I've read a lot of books and basically all of them said "#1 Best Seller", but how could that be?

Hi Abeer,

a bestseller is the one that was number one selling on Amazon or in bookstores whether it stayed in this ranking for 1 day, 1 week , a month or more it will be ranked as bestseller.

Also it is in different categories, for example 4 novels can be bestsellers at a time but one as a thriller, other as a romance another one in the young adult section and so on...

I hope my answer was helpful:)