I think 10 acres is a good minimum for you. Like @walkinthewalk said, you can get by with less but then you are going to make up for it in hay and pasture management. We had 3 horses on 10 acres (not all pasture but I can't remember how much was under fence) at our last place and even with that we had to be careful with the pastures. Now he have 5 horses on a 23 acre pasture and they can be on it year round without it needing rested. Needless to say the 2 in the 13 acre pasture don't even make a dent in it but those are the actual sizes of their pastures and does not include the corrals around the barn that they also have access to.