A rule of thumb is that you need four acres of land per horse. Of course, this is not a strict number—you can have more or less—but it all depends on how you want to maintain your horses. Four acres is a good amount of land for horses that graze the pasture for around 80% of the day.
The amount of land you need also depends on what you’ll be using it for. If you train sport horses, you’ll need a property with nice facilities, like arenas and barns, but you may not need acres upon acres of land.