Valency of Chlorine is 1. Why?

Valency of Chlorine is 1 because it needs 1 electron to complete its octet (a shell of eight electrons). Valency is defined as "The combining power of an element" and Since, chlorine needs 1 electron to complete its valence shell, Its valency is 1.

if two electrons have the same n, the same ℓ, and the same ml, then they are in the same orbital. the pauli exclusion principle states that no two identical fermions (a fancy word for electrons and other subatomic particles like electrons) may occupy the same quantum state in an atom simultaneously.