The number of chlorine atoms used is found by finding the compound(s) where chlorine is found in either the reactants. In this case, FeCl2 is the only compound with chlorine in the reactants so we will focus on it.
Then multiply the subscript of the chlorine (2 in this case) with the coefficient of the compound (3 in this case). 2*3 = 6
So there are 6 chlorine atoms used.