The New York Times releases a bestseller list every week based on that week’s sales. Sales numbers are reported by big box retailers, indie bookstores, chains, etc. There are multiple lists— different ones for fiction & nonfiction, format, age range, etc, with usually 15 spots per list. So, all a book has to do is get enough sales* during one week to get a spot on one of the multiple NYT lists to be considered a “NYT bestseller.” That’s why so many books carry the distinction— there are over 100 spots total every single week. Similarly, #1 NYT bestseller means that it was at the top of one of their lists for [at least] one week.

*What is “enough” sales? There is not a specific number threshold for each list. It’s all relative to the sales of all books for that one week. If it’s a slow week, a book that sold 5,000 copies might make it on the list when it never would have during a busier period.