Al + O2 -> Al2O3
Balance it:
2Al + 3O2 -> 2Al2O3
So you need 2 Al and 3 O2 to make 2 Al2O3 (aluminum oxide).
I'm going to assume you have all the O2 you need.
Since 2 mols of Al is needed to make 2 mols of the product, it's a 1:1 ratio. You get as much aluminum oxide for as much aluminum you burn.
So 12.5 mols if there is not a lack of the O2.
