Through my professional / personal network, I know someone who advertises himself as being a “Best Selling Amazon Author in XYZ category.”

It is semi niche, but I did some ballpark math, and about 72 sales rapidly would put him in the top spot for that niche.

That number sounds about right when he’s mentioned the gross $ sales of his book.

Pretty common for authors to get people to pre-order their books so when they go on sale they top the chart for that day (the book's release day) in their category.