IF I BUILD A 8X12 PLYWOOD BOX AND FILLIT WITH 8X12 BLOCK STYRO INSIDE OF BOX HOW MUCH BUOYANCY WOULD I HAVE ?

What Mike said is pretty much spot on, it is a question of Displacement. If the vessel is 12' long, 8' wide and has a Draft or depth of 2', it should float. The styrofoam is just cargo. Some boat builders use foam and such stuff to give a vessel Positive Flotation if it is swamped or pitch poles or is knocked down, but that only is there to save the people and the vessel, there is no guarantee that it will work, but is there.