Decking fastening schedule?
It would be best to use two. If you make sure your deck boards are cupped down you would think that only one screw would be fine, to stop the board from cupping up in the center. I have seen wood cup in some extremely odd ways so this is not always the case. Two per board/joist intersection will ensure you do not need to mess with it in the future. Or you can always just use one and wait for the weird boards to cup :) I prefer low maintenance and the screws shouldn't be too pricey. Good luck!