How do you know how much food to give to a horse?

Usually the apatite of your horse can answer that for you. But it depends on the weight of the horse and what type of work the horse does. Also on what type of feed you give your horse. If there is a lot of food left after your horse walks away from it, and you notice your horse getting fatter,you need to cut back a little. If your horse tries to eat everything in sight that is eatable and you can see the outlines of its ribs and hip bones, you need to feed more.