Question

What is the Oxidation number of Sulfur (S)?

Answer 1

Zero, because sulfur is an element.

Answer 2

The oxidation number of all elements in the elemental state is 0. So typically you need to know the compound in which the element is before you can calculate the oxidation number.
S is typically -2 in sulfides (FeS).

Answer 3

Preetha is right and made a fuller answer than mine.

In compounds, sulfur has one of the greatest varieties of oxidation states, including some that lead to uncommon results (peroxodisulfate ion for instance) where you must draw the ion's structure to understand what happens.

S is also -2 is another famous sulfide: H2S
But it can be +4 is SO2 for instance.