That is a good question! Sulfur is in the 3rd period of the periodic table, and it has 6 valence electrons because it is in group 6 of the main group elements. Organic chemists call sulfur with 6 bonds, as I've shown in this video, an "expanded octet". As you've observed, in this case, sulfur has 12 electrons in its valence shell. This is only allowed with period 3 elements and greater. Some chemists prefer to show the sulfur with only 4 bonds. If I did that in this video, there would not be the two S=O double bonds. Instead, there would be 3 S-O single bonds, with a 2+ charge on the sulfur, and 1- charges on the two oxygen's that used to have double bonds to sulfur. In this way, there would be no expanded octet. You can see an example of this way of drawing on this webpage: www.chem.ucla.edu/~harding/tutorials/resonance/draw_res_str.html Structures I and J show the different ways of drawing sulfur, with 4 bonds or with 6 bonds.