If I have 12 moles of C2H6, how many moles of H2O are produced?

I assume you mean how many moles of H2O are produced when C2H6 is burned, right?

First ask yourself this - if you burn an atom of C2H6 how many atoms of H20 could be produced (assume the oxygen is freely available). From that you should be able to tell how many moles of H20 come from a mole of C2H6.