The bonding number n of a skeletal atom is the sum of the total number of bonding equivalents (valence bonds) of that skeletal atom to adjacent skeletal atoms in a parent hydride, if any, and the number of attached hydrogen atoms, if any. Examples:
SH2: for S, n = 2;
SH6: for S, n = 6.
Source:
Blue Book Guide, p. 21 (https://doi.org/10.1351/pac199971071327)
See also:
PAC, 1984, 56, 769. 'Treatment of variable valence in organic nomenclature (lambda convention) (Recommendations 1983)' on page 774 (https://doi.org/10.1351/pac198456060769)