The first step would be to filter based on the the Clients_clientid, and depending on how you are going to filter (either by a url string or a session), in Dynamic Web Charts on the Data tab, there is a filtering and sorting options where you can select the column to filter. In your case that would be your Clients_clientid column. And then under the X Label there is a Group By option which you need to add, and then under the Y Value there is a calculation drop down list and from that you select the Count function. That should give you how many products a user have sold.
