How can I limit the number of items being selected from parameter value dropdown list box?

How can I limit the number of items being selected from parameter value dropdown list box?

I am creating a CR report. This report has a parameter which allow multiple values. There will be hundreds items available for users to choose but a user can only choose no more than 20 items to run the report. Is there a way to do that in CR ? or when a user selects more than 20 items, can I let CR popup a warning widow?

Thank you!