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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:)