Different people do it different ways; however, I think that since S has been found to exist as the S8 atoms to the molecule, the correct way is to write it as
8Fe + S8 ==> 8FeS
8Fe + S8 ==> 8FeS
8Fe + S8 ==> 8FeS
Iron has a valency of +2, which means it can form compounds by losing two electrons. Sulphur, on the other hand, has a valency of -2, meaning it can gain two electrons to form compounds.
Iron and sulphur combine in a 1:1 ratio to form iron sulphide. This means that for every 1 atom of iron, we need 1 atom of sulphur. However, in nature, sulphur exists in the form of S8 molecules, where eight sulphur atoms are bonded together in a ring-like structure.
So, in order to balance the equation correctly, we need to consider sulphur as S8. The balanced chemical equation for the reaction between iron and sulphur to produce iron sulphide (FeS) can be written as:
8 Fe + S8 → 8 FeS
In this equation, 8 atoms of iron react with 1 molecule (S8) of sulphur to produce 8 molecules of iron sulphide (FeS).