Assuming your reaction is balanced, you need three moles of copper (II) chloride hydrate for every two moles of Al.
You need to be given the number of grams of each, so you can calculate the number of moles of each that are present. Then, you look at the ratio of each, so it it is 3:2, then the reaction has no limiting reagent. But let's say you have 4 moles of the copper compound, and two moles of Al. Then, Al is the limiting reagent. And if you have two moles of Al, you will only get two moles of AlCl3 and 3 moles of copper. So if you were asked for the theoretical yield of Cu, it will be 3 moles, and that relates to a certain number of grams.