Take the molecule and split it up as follows:
Na2SO4 ----> 2Na+ + SO4^2-
Count the moles on the right and divide by the moles on the left, which is 2 +1 = 3
This is fairly important when your using colligative properties...if the compound dissociates in solvent then you need to know the vont-hoff factor...in this example our vont-hoff factor is 3