Users may occasionally want to choose numerous values from a list or a dropdown menu. Those many parameters should be taken into consideration while displaying the report or the page’s content.
You’ll learn how to create a report with numerous values that users can choose from the provided parameters, in this post.
Allow Multiple Values:
Expand the Parameters node in the Report Data pane, then right-click the dataset parameter-generated report parameter and select Parameter Properties.
To allow a user to select more than one value for the parameter, click Allow multiple values under the General tab.

Hope this helps.