Shipment Number based on the po header id information:-

select distinct rsh.shipment_num from RCV_TRANSACTIONS rt,PO_LINE_LOCATIONS_ALL pll,RCV_SHIPMENT_HEADERS rsh where rt.PO_LINE_LOCATION_ID = pll.LINE_LOCATION_ID
and rt.SHIPMENT_HEADER_ID = rsh.shipment_header_id
and pll.po_header_id = ‘Your PO Header ID’
You can know the po_header_id of any particular PO Order Number.
select po_header_id from po_headers_all where segment1 = ‘Your PO order Number';