A classical rule for describing the electronic configuration of atoms in molecules: the maximum number of electron pairs that can be accommodated in the valence shell of a first-row element is four. For the second and subsequent row elements there are many exceptions to this rule.
Source:
PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1949 (https://doi.org/10.1351/pac199971101919)