This calculated field returns the number of sub-tasks, which are open (in status category To Do or In Progress).
Create a new Calculated Number Field (by JWT) custom field and name it properly.
Providing a description will help you to identify what the custom field does but this step is optional
Add the following expression:
Add the field to the relevant view screens.
Remember: All calculated fields are read-only and cannot be manually updated on a create or edit screen.
Check the results
Once configured and added to an issue screen (a parent issue ), the result will look like this: