For SO_4^(2-), a reasonable Lewis structure to accommodate the 32 electrons is.....(O=)_2S(-O^(-))_2, and clearly this is a derivative of the parent "sulfuric acid", .....(O=)_2S(-OH)_2. Given this Lewis structure, there are 6 valence electrons associated with the doubly bound oxygens (and hence these are neutral), 6 valence electrons with the central sulfur (and hence a neutral sulfur), and 7 valence electrons associated with each singly bound oxygen atoms, and hence these centres bear the 2 required formal NEGATIVE charges.
Note that all the oxygen atoms are equivalent, and we could even propose a Lewis structure of ""^(2+)S(-O^(-))_4.....(O=)_2S(-O^(-))_2 is a more conventional representation.
