Depends on your location and circumstances.

IF you are relying on your land to feed your horse than it is 1 to 1.5 acres of grass pasture per horse.

Otherwise like Cheryl said it depends on your local zoning laws and if you live in an HOA how many they allow.

Here in AZ where grass is limited we supplemental feed with hay. Some areas only allow two horses per acre, where some areas have no restrictions.

Look at boarding facilities. It may be a 5 0r 10 acre parcel and have 30 horses on it.

I have 1.3 acres and we have 4 horse on it. They have a turn out area that is 70 ft wide by 220 ft long. Which they are out in most of the time. When the weather is bad they come into the barn and have a 16x16 foot stall. They are ridden a few day a week normally. I wouldn't want more than 4 on our size lot, I feel it wouldn't give them enough space.