I notice that every time I see items in shopping carts, I put a couple of these items on sale or maybe all of them. Then all of the items that were previously in shopping carts disappear. So now that these items were put on sale. No one has them in their shopping carts any longer?
When you run a sale it no longer displays the number of people who have the item in their cart, because there is only a limited amount of space and the sale takes precedence. The items are still in the cart (unless Etsy clears the carts and puts the items in "saved for later" which they do approximately once a month) and when the sale is over, they will show in carts again.