With seven valance electrons Chlorine needs one more electron to achieve a stable electron configuration The most common oxidation number for Chlorine is -1 This means that the electron density of another element has a greater probability of being near the Chlorine than the other atom.
Cl = -1
However Chlorine could bond with an element with a higher electron negativity than Chlorine and the electron density due to bonding could result in the electron (s) having a greater probability of being near the other element such as Oxygen
Cl = +1
Cl = +3
Cl = +7
So Chlorine always has 7 valance electrons. The sharing of the these valance electrons can result in multiple oxidation numbers