What is the oxidation number for Sulfur in the compound Na2SO4?

The oxidation number of sulfur is +6
Sodium sulfate(VI) is ##Na_2SO_4##
It contains 2 ##Na^+## for every ##SO_4^(2-)## ions.
Oxygen has oxidation number -2 in the sulfate(VI) ion.
The on the ion is equal to the arithmetic sum of the .
So oxygen contributes 4 x -2 = -8
Since the net charge is -2 the sulfur must contribute +6 since 6 + (-8) = -2
So the oxidation number is +6 hence the name sulfate(VI).