Nothing prohibits to give a definition of the multiplicity of bond:
the multiplicity of bond is the energy of bond expressed in dimensionless units.
Hückel rule (4n + 2) for aromatic systems can be written in a different form, in the
form of 2n where n - unpaired number. So, we have: 2, 6, 10, 14, 18, etc. This is
also true for the electron shells in the atom and aromatic systems. The principle of
the interaction of fermions always one, everywhere.