The amount of area/horse will vary depending on location and type of grace and so much more. Around here I can keep 5 horses on about 4-5 acres. However I do quite a bit of work to keep every thing nice. Like no horses are out of the grass this time of year. They are all dry lotted and hayed.
In the summer they go out in the grass pastures at about 7-8 am and stay out there until about 11pm. Then they come into the dry lot areas and have a bit of hay. Really saves the grass that way I have good pasture up until almost nov.
Through out the summer I rotate pastures. Clean the areas that needed mow the pastures and drag them to brake up what ever did not get cleaned up. I also use Lime in the areas where they make the most manure and keep the pastures fertilized properly and at times that means taking soil samples into the county extension for testing. This really pays off bit time.
Good maintenance is the key really no matter how many horses on how much land