Hi Ann.  Canned foods should not be thickened before canning unless the recipe calls for it.  An example would be apple pie filling.  It is important that it is not too thick so the heat can penetrate throughout the jar making it a safe product to can.  Clear Jel or corn starch can be used AFTER opening the jar to reheat the corn before serving. Depending on the thickness desired, I would say experiment with the amount between 1 teaspoon to 1 tablespoon again after opening.