One important piece of information that we can get from chemical equations is the molar ratio between elements in a reaction. The coefficients actually tell us how the number of moles of one molecule can translate to the number of moles of another molecule. In this case, we know that 6 moles of HCl can yield 2 moles of AlCl3 based on the coefficients (when there is an excess amount of aluminum). Therefore the ratio of HCl:AlCl3 is 6:2. Using this same logic, if we have 60 moles of HCl (10x6 moles), then we must be able to form 20 moles of AlCl3 (10x2 moles).