Whew! You scared me! You have three more horses, which, in the long run, would mean we'd need about 85 bales or so, and although our fields produce fairly well under normal conditions, we wouldn't get quite that much. Then I realized you feed 700 pound bales, and ours run 1,000 - 1,200. If I'm figuring correctly, we'd need about 60 - 65 bales. We got 45 last year, but it was the first time they'd been cut in years. We also only cut the back half once because we had to repair a ditch gap before we could get back there. Now they've been fertilized and overseeded, so we should get more. Maybe enough we won't have to buy any...I hope.
A lot of our farmers claim that their hay is 1,000 lbs, but if my daughter and I can roll it by hand, it's nowhere near 1,000 lbs, lol. I'm lucky if I can get 800 lb bales.
Free choice, 7 horses would go through about 5000-6000 lbs of hay a month, if they don't have any grazing. If you have decent pastures, you don't need to feed much, if any, hay during the summer. If they have even just minimal grazing, they'll eat less hay. Figure about 25-35 lbs a day of hay per-horse for highest consumption.