https://doi.org/10.1351/goldbook.B00703
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: $\ce{SH2}$: for $\ce{S}$, \(n=2\); $\ce{SH6}$: for $\ce{S}$, \(n=6\).