3 MgCl2 + 2 Al = 3 Mg + 2 AlCl3
Since we know that we have 12 moles of MgCl2 and enough Al, we don't need to figure out the exact amount of Al.
3 MgCl2 / 2 AlCl3 = 12 mol MgCl2 / x mol AlCl2
cross-multiply
3 MgCl2(x mol AlCl2) = 12 mol MgCl2(2 AlCl3)
3x = 24
x=8
8 moles AlCl3
Alternatively, we know that we have 12 moles of MgCl2, and a coefficient of 3. This indicates a multiplier of 4 for the entire equation, meaning 2 AlCl3 multiplied by (4) = 8 moles