Valence is the number of valence electrons or the number of free positions on the valence level of an atom. If an atom has exactly one electron or exactly one spare place on its highest energy level, it has the valence 1. Such an atom is only able to form one single binding to another atom Atoms with two valences have the choice to form one binding with both valences to another 2-valent atom or two bindings to two atoms with one single valence.

As an atom cannot have more than 8 valence electrons the maximum  number of valences is 4.