DECLARE
    v_total   NUMBER;
    v_message VARCHAR2(300);
    v_count   NUMBER := 0;
    v_label   VARCHAR2(50);
BEGIN
    v_label := Get_item_property('SELECT_ALL', label);

    IF v_label = 'Select All' THEN
      first_record;

      WHILE :System.last_record != 'TRUE' LOOP
          IF .....then
            :SELECTED_FLAG := 'Y';
            v_count := v_count + 1;
          END IF;

          next_record;
      END LOOP;

      IF :System.last_record = 'TRUE' THEN
        IF ....then
          :SELECTED_FLAG := 'Y';
          v_count := v_count + 1;
        END IF;
      END IF;

      v_label := 'Deselect All';

      Set_item_property('SELECT_ALL', label, v_label);

      first_record;

      fnd_message.Set_string('Total '||v_count||' Records selected.');

      fnd_message.show;
    ELSIF v_label = 'Deselect All' THEN
      first_record;

      WHILE :System.last_record <> 'TRUE' LOOP
          IF :SELECTED_FLAG = 'Y' THEN
            :SELECTED_FLAG := 'N';   
          END IF;

          next_record;
      END LOOP;

      IF :System.last_record = 'TRUE' THEN
        IF :SELECTED_FLAG = 'Y' THEN
          :SELECTED_FLAG := 'N';
        END IF;
      END IF;

      first_record;

      v_label := 'Select All';

      Set_item_property('SELECT_ALL', label, v_label);
    END IF;
END;