We have difficulty in implementing this request. We will consider this request in the future.
For now, I'm suggesting a workaround for you.
Using Employee Import Facility, you can import employees from an excel sheet. With excel it's easy for you to calculate Employee Numbers as described in your post. The video in the link below has details on how to import employees in bulk.
So, if you keep an excel file for employee import, then you can update the file and import it to Attend HRM when you want to add new employees.
Hope this helps!