How much feed does a 12.2 hand horse need daily?

This will depend on the individual horse. Some horses for one reason or another will need more or less food than others of his size. The type and quality of his feed is just as important as how much. As long as your horse is healthy and worm free and his teeth are in good condition I would try 1 to 1 1/2 flakes of a good oat or oat mix hay twice a day. If he is out in pasture and has plenty of grass there you may want to cut it down a little. Be careful about how much green grass he is getting from his pasture. It is very easy for horses of his size to over indulge on rich green grass and end up with laminitis (founder). Sometimes finding the right feed program for a horse is a trial and error operation. Watch him closely if he has dropped weight or has gained weight. Adjust the amount depending on how he is looking. Horses his size are usually easy keepers.