By default, both of your user parameters appear in the Runtime Parameter Form. In reality, you only want users to set CUTOFF_CNT. You do not want them to be able to set INCR_CNT (the amount by which your counter is increased for each record). To prevent users from seeing INCR_CNT on the Runtime Parameter Form, you will build your own Parameter Form.
Choose Tools > Parameter Form Builder.
In the Parameter Form Builder dialog box, scroll down the list of parameters until you find INCR_CNT.
Click INCR_CNT to deselect it.
Change the label for CUTOFF_CNT to:
# of Top Customers:
Click the Run Paper Layout button in the toolbar.
In the Runtime Parameter Form, type a value for # of Top Customers, then click the Run Report button in the toolbar. You should now see as many records as you asked for in the Runtime Parameter Form and they should be in order from largest total purchases to smallest total purchases.
As an additional exercise, you could now change the Initial Value property of CUTOFF_CNT to see its effect on the Runtime Parameter Form.
Save your report.