The average you often see is 2 per horse. But, that is dependent on your area. The ag extension agent for your county can give you the best idea. Add an acre for a barn/storage /dry lot and then room for your house and drive. I'd say 5 is the minimum if you have good grass most of the year and adequate rainfall to keep things green and growing. You don't say where you are at but you'll need to figure in hay. They can tell you about how long you'll need to feed over winter (late fall/early spring). You'll also want to look at your state laws for fencing livestock. Depending on where you locate you may not be able to use electric along a road. Now if the pasture area is behind your house you may not have restrictions and if the horse is acclimated and respects it then it works well. As for voltage you want a livestock charger. As long as it is rated for large animals (cows, horses, around here the odd camel or few) then you would be fine. Best is one that plugs into a source but I have used solar successfully. You just need to make sure you have a good sized battery to store power.