This operator returns the number of patches in the subdivision hull. Each face in the hull will generate one or more patches, so this number will always be greater or equal to the number of coarse faces in the hull.
face_id input is specified, the number of patches generated by the given face will be the output. For example, if the input face is a triangle, there will be 3 patches generated. If the input face is a quadrilateral, only one patch will be generated.