Reporting highlights of number of items selected in a multi pick list

I'm trying to create a highlight on a report that counts the number of unique selected items in a multi pick list across multiple sub projects.

Case: Have 5 sub projects under a parent project and in each, there is a multi pick list on the property card to select multiple departments.

End Results: If across the 5 sub projects, there were 3 unique departments selected, the highlight would show 3.