The reaction of cesium fluoride with xenon hexafluoride is CeF + XeF6 -> CeXeF7 and the reaction is balanced as written. So the mole ratio is 1:1:1. We are given 12 moles of CeF and 14 moles of XeF6 are reacting, but after the 12 moles of CeF react completely, the reaction will stop as we have run out of one of our reactants. So only 12 moles of CeXeF7 will be produced.
