Thanks Don.
I put up my 1/2" ply over the stock luan that was in the trailer from the factory. I used the self tapping (#8) screws and placed them into the uprights as you said. Mine are also 16" on center. My E-Track will just be screwed to the ply and may or may not happen to line up with the upright. I was thinking #10 or #12 as well. A screw too big will not have many threads in the wood considering we are only going in 1/2". I was asking how many screws to use not to be cheap but to be sure I had enough.
Thanks for the input.
Dom