Oxidation number means that the number of electrons either lost or gained by an atom of an element during compound formation. The oxidation number of chlorine are +1,+3,+5,+7 and -1. However for Chlorine gas, the oxidation number (Cl2) is zero. Chlorine will have different oxidation number in oxide forms.
In Cl2O oxidation no. of Cl is +1
In Cl2O3 oxidation no. of Cl is +3
In Cl2O7 oxidation no. of Cl is +7
In HCl oxidation no. of Cl is -1
In NaClO oxidation number of Cl is +1