Go to report view, right-click on the OrderDetails table and select New measure from the Fields Pane (top-right of the report page).
Enter the following DAX then hit Enter:
Count of Orders = COUNTROWS('OrderDetails')
A new field is created called Count of Orders and it should have a calculator icon next to it.
Select the Count of Orders from OrderDetails. It should be highlighted in a grey shade.
Under Measure Tools tab > Select the comma as highlighted in red square