Hi Gail and Audrey,
I am not sure about this, but I think the best sellers are the cards that have had the most sales in the past 30 days. By most sales, I think it is not based on the quantity of cards sold, but the individual sales. To be clear, a card has been ordered by 20 Different customers makes the best sellers not One customer ordering 1000 cards.
I used random numbers for my example. There is no magic number, it is just the most individual orders of a card, not quantity.
I don't know if this makes sense or not! I know what it means in my head!