Answer: Relevant RDA rule is given below from RDA Toolkit.
RDA rule 126.96.36.199 is for "Number of Bibliographic Volumes Differing from Number of Physical Volumes"
According to this rule it should be given in MARC as:
300 $a 5 volumes
500 $a 8 bibliographic volumes in 5 physical volumes
Please note that 188.8.131.52 is revised. Earlier it was numbered as 184.108.40.206.
Note: Serials are exception.