To see how often instructors subbed in a specified timeframe, you can run the Instructor Payroll Report and filter the report by the instructor.
Navigate to Reports from the left panel > Operations > Instructor Payroll > View.
Select the timeframe from the date picker > (optional) apply More Filters.
Export the report.
Open the report > navigate to column D > create a filter to only display Is Substitute: True.
Then filter by instructor for the total substitute count.