If this is the reaction:
8H2 + S8 -> 8H2S
For every 1 mole of S8 the reacts, it requires 8 moles H2 to form 8 moles of H2S.
As an example, if it is required to form 3.5 moles of H2S then, 3.5 moles of H2 would be needed and 3.5/8 = 0.4375 moles of H8 would be needed as well.
