X-Cart:Bestsellers list not shown

First of all, make sure you have "Bestsellers" turned on on the "Modules" page. 'Bestsellers' is an automated list. It will display the most popular products that have ever been ordered in your store. To start it off, make a test order and buy items that you want to see on this list. As soon as you change the orders statuses to "P" ("processed") the purchased items will be added to the Bestsellers list.

You can change the number of products listed in the Bestsellers section via "General Settings" -> "Modules options" -> "Number of products in bestsellers list" field.