You can obtain a count of students who have earned a "Diploma" using a Custom Query. This query is in Master Reports | Custom Queries (or browse the extended list and subscribe). This report lists all students where the Diploma Date field on the Reg Form has a date that is within the date range of the report. You can also restrict the results set to just members of a specific Customer Group.
Notes:
1. This does not look at the Diploma Date found on Credit Programs.
2. This does not require the Diploma Earned checkbox on the Reg Form to be checked; just the date to be within the date range.