It depends on where you live. Where I live, you need at least one acre per horse. I am planning to move soon, and the place I am looking to move only needs 1/2 acre per horse. In my opinion, 1/2 acre is a little small.
I'm currently boarding my horse at a family stable. They have five acres total, this includes: the house, the pool, the barn, the pasture, and the riding ring. They have four horses, one foal, and one pony. The land is quite over grazed, but it is within the legal limit for that area.
You have to ask around in your local area to see what the minimum acreage per horse is. The minimum acreage to own a horse varies from county to county.