How do you retrieve the number of related instance from PACS server using pynetdicom?

[Ikhwan Salleh]

    ae = AE(ae_title=b'some name')

    # Add a requested presentation context

    ae.add_requested_context(PatientRootQueryRetrieveInformationModelFind)

    # Create our Identifier (query) dataset

    ds = Dataset()

    ds.StudyDate = str(start.date()).replace("-","")

    ds.QueryRetrieveLevel = 'SERIES'

    ds.Modality='CT'

    ds.SeriesNumber='501'

    # Associate with peer AE at IP 127.0.0.1 and port 11112

    assoc = ae.associate('some ip', 4100,ae_title=b'some pacs server name')

    if assoc.is_established:

        print("Connection Established")

        # Use the C-FIND service to send the identifier

        # A query_model value of 'P' means use the 'Patient Root Query Retrieve

        #     Information Model - Find' presentation context

        responses = assoc.send_c_find(ds, query_model='P')

        

        for (status, identifier) in responses:

            # If the status is 'Pending' then identifier is the C-FIND response

            if status.Status in (0xFF00, 0xFF01):

                print(identifier)

                identifier.StudyInstanceUID

                identifier.SeriesInstanceUID

                identifier.StudyDate

                identifier.NumberofStudyRelatedSeries

the problem is python return me error datasets object has no attribute Numberofstudyrelatedseries