When I do my figures, I start at 10 average sized bales a month per horse and go up or down from there. In the winter, they will be eating more and the wind and mud and in the summer they eat less and it is pretty easy. So for planning, 10 bales a month per horse, average horse,1000-1100lbs, average keeper, light riding. If you will be storing it outside, then you have the weathering issues depending on tarps, etc. There are lots of variables for sure and I like to keep extra on hand.
This translates into about 20 lbs of hay a day. If you are keeping 3 horses, they are going to eat a bale a day in general. Double what you had down. Real easy keepers can get by on 10 lbs a day but then you might have other issues. In horses, hay is not just a food; they are mentally and physiologically designed to eat throughout the day/night and cover some distance to do it. They are browsers, like deer. So tossing one tiny little flake in the am and pm can set them up for lots of health and behavior issues. It really depends on the horse and what you are feeding and how often they get out to go do stuff.
It is something that you will get a feel for when you get them and see what their individual needs are. You might be able to feed big bales free choice in a feeder or you may have to set up feeding pens. Just be flexible. IMHO
