How do you find out the valency for each element?

Valency (oxidation number) is characterizing the ability of an atom to combine with another atom.

 It is based on the number of electrons that atom participates in chemical bond formation and varies depending on the atom and chemical group.

Valency is determined by the number of electrons involved in chemical bonds.

 The number of main group indicates the maximum valency of elements. Valency may relate to hydrogen and oxygen. Valency of elements relative to hydrogen increases in period from group IA to group IVA, and then decreases. For elements of groups V-VII ,valency to hydrogen is determined by using the relationship: valency = 8 - no. group.

Electro valency represents valency of elements which are converted to ions and  is equal to the number of assigned or accepted electrons.

Covalency represents the valency expressed by the number of  electrons which an atom puts together with electrons of another atom. Covalency is denoted by roman numerals, written in parentheses, on the right side of the symbol. For example: H (I), A (II).

Valency of elements is considered between two elements.