Question

Why sulphur written as s-8not s-2

Answer 1

As it has this kind of structure.
In nature there are also few structures like S6

Answer 2

 So comparing one S8 and 4S2 molecules, the S8 is more stable as S8 has 8 sigma bonds while 4S2 has only 4 sigma and 4 pi bonds. And since pi bonds are weaker that means the 8 sigma bonds are more stable than the 4 sigma 4 pi bonds.