You need 8 Al2S3 and 3 S8 to balance the sulfur atoms (S), so that both molecules contribute lcm(3, 8) = 24 atoms of S.
Then there are 16 atoms of aluminum (Al), so you need 16 Al among the reactants.
So, the balanced reaction is
16 Al + 3 S8 => 8 Al2S3
