There's probably an amount of carbs at which the body decides to switch to carb-burning preferentially and I have noo idea what that is. It's probably highly individual. Could be one roll could be four. But I doubt that eating bread will help you stay in ketosis even if your BG goes down, somehow. Lower BG just means that some has been cleared out of the blood, the ketone measurement is what tells you whether you are or aren't in ketosis.