Hi Jana,
You need to work on your logic a little bit.
1. You need to keep the original request of no of opening.
2. You also need to keep the latest no of opening in the request in a separate field than point 1.
3. When you saved the form and calculate the No of opening in request, calculate it as :
Original Request (pt.1) - Query count of the selected candidates.
4. Update point 3 above in the latest no of opening field (point 2.)
Regards
Edy