It depends on the type of feed you are giving them. It is important to read the feed label and discuss your horse’s needs with a vet.
1.5-2% of their BW in dry matter (15-20 pounds of hay and more than that of grass) and about .5-1% of their BW in concentrates (5-10lbs) for an average 1000 pound horse.
I've always been told a minimum of 1% roughage (like hay), so for a 1000 lb horse that's about 10 lbs of hay. Most people I k ow, however, feed about twice that for maintenance and higher for working horses and those that need to gain