See attached XSN. Default Value is set on Count fields with filter condition
1) create Count field, and open its property
2) Click fx in Default Value.
3) Click Insert function and select Count
4) Click "insert..." and select repeating folder
5) Click Filter Data, and set condition that Check Box is equal to TRUE
6) make sure "refresh value when formula is recalculated" is checked
7) test it in preview
hope this would give you an idea...