According to the reactivity series, this reaction will not take place since Al cannot replace the Mg in MgCl2. However, if your teacher wants you to find the mole amount anyway regardless, this would be the solution:
Based on the information provided, you can write the following reaction:
MgCl2 + Al --> AlCl3 + Mg
But this reaction is still not balanced, so we have to do that by adding numbers in front, also known as coefficients.
3 MgCl2 + 2 Al --> 2 AlCl3 + 3 Mg
Now, you can look at the coefficients in front of the reactants (MgCl2 and Al) to find that for every 3 moles of MgCl2, 2 moles of Al also participate in the reaction. With this in mind, we can use the given amount of 12 moles along with dimensional analysis to get the answer:
Hence, your answer is 18 moles of MgCl2.
I hope that helps!