Since there are three chlorides on the product (right) side and only two on the reactant (left) side, you need to multiply Cl2 by 3 and multiply FeCl3 by 2 to balance the chlorines. Next, to balance the irons, you need to multiply the Fe by 2. So you get:
2Fe(s) + 3Cl2(g) ----> 2FeCl3(g)
If you use the mole-to-mole ratio between the chlorine gas and iron (III) chloride, which is (3:2) based on the coefficients, then:
14 mol FeCl3 * (3 mol Cl2 / 2 mol FeCl3) = 21 mol Cl2
