The oxidation number of chlorine (Cl) in Cl2 is 0. This is because in its elemental form, a molecule consists of two atoms of the same element. When two chlorine atoms bond together to form Cl2, neither atom has a charge, thus their oxidation states are both considered to be zero.
To elaborate, the oxidation number is a concept used to determine the degree of oxidation of an atom in a chemical compound. In elemental forms, the oxidation number is always zero. Therefore, since Cl2 represents elemental chlorine, the oxidation number remains 0.