To get to the item number, you need to loop at the table returned by the GET_ITEMS method, and call another method, GET_DATA.
Something like this:
DATA: ls_poitem_obj TYPE purchase_order_item,
lo_po_item_handler TYPE REF TO cl_po_item_handle_mm,
ls_poitem TYPE mepoitem.
LOOP AT lt_poitem_obj INTO ls_poitem_obj.
lcl_po_item_handler ?= lw_poitem_obj-item.
CALL METHOD lcl_po_item_handler->if_purchase_order_item_mm~get_data
RECEIVING
re_data = ls_poitem.
ENDLOOP.
Structure LS_POITEM should contain the item number, field EBELP.