Horses, in general need 1-3% of their body weight in hay. However variables of hay quality, activity level of the horse, housing available, current body condition and other feed available come into play. So there is not a hard and fast answer.
Many people want to know how many flakes or bales their horse should eat- this is a huge issue as flake size and weight, and bale size weight can be so variable. I personally have small square bales of alfalfa at my house. I have some stemmy alfalfa bales that might weigh 35 lbs each, and some 3 strand leafy alfalfa bales that weigh 110 lbs each.
Horses need roughage in their diet- and this can be supplied by pasture, hay, or many other alternatives such as beet pulp, hay pellets and complete grains such as a senior horse feed.
If you would like to email me with the specifics of your horse, Id be happy to offer a few starting points to help!