As your query is answered to be sorbic acid, I thought of giving you an approximate quantity.
But before that, you can use the same preserving methods used for bread instead. Usually, if required for three or four days, a plastic bag is wrapped around bread, and is stored in a cool area. Or in freezer. It can later be thawed by a microwave oven.
Generally half-cooked foods are dangerous to be preserved.
You can use both sorbic and ascorbic acids to preserve chapatis. Ascorbic acid is Vitamin C. So you can crush any vitamin C tablet and apply it on the food to preserve. If you find that hard, dip the chapati in honey. Honey never goes waste. Then wrap the chapati in a paper and store in a cold place.
The stronger a light shines the darker are the shadows around it.