The key is 'include details about every session' in Client Session report
One way to get this data:
Then you're going to want to create a client session report
Include session data by ssid (list)
Include details about every session -> this is the important part
Set time range for 3 weeks ago to now (3 weeks ago is limited by amount of time historical data retention is set)
Save and run
Once the report is generated
Export the details about every session to CSV
Then you can manipulate in Excel or Spreadsheet to sort by Device Name, SSID, time frame and get counts of rows.
Caveats:
If you have a large number of devices / folders -> you may want to create a separate client session report per folder since the reports will get large
This is probably something worth raising a feature request about as well. This can be done from the Ideas Portal on the Support Page.
Clients = unique clients that were present on the SSID (but won't matter if you're only parsing the detailed sessions table)
Sessions = unique sessions with a start and end association event -> multiple sessions can represent a single client in a time period.