First we will need to the complete chemical reaction involving sulfuric acid and alluminium.
2Al + 3H2SO4 -----> Al2(SO4)3 + 3H2
From the reaction
2 moles of Al react with 3 moles of H2SO4 [ That is is 2:3 ratio]
18 moles of Al will react with 27 moles of H2SO4.
(18 * 3)/2 = 27 moles of H2SO4.
Therefore 18 moles of Al will require 27 moles of H2SO4 to react.