If you open the "Consumer Goods Sales" example application that comes with Qlik Sense. Go to the sheet called "Sales Rep Performance" and then click on the EDIT button to inspect how the various tables and charts are set up.
Click on the lower left chart called "Toal Revenue =..." :
Change the options under the section DATA and Dimension Sales Rep, Click on Limitation and select Fixed Number - click Top and type 10 in the fx input field. Finally you can click DONE to go save your changes.
You can select the relevant dimension for your counting and as a measure you would use the Count(DISTINCT .....) function.