Around here, with clay-y soil but optimal weather conditions for growing grass, you need a VERY minimum of 3 acres per horse for grazing 24/7 all but 9 months of the year- other three months only a half day of grazing. That's with very careful management of fertilizing, rotating, and weed spraying.
Now.. are you thinking that one acre per horse will sustain a half day of grazing daily? I would re-think that, either get a bigger property or just have an acre dry-lot and a few small little grazing areas for a couple hours of grazing every day or two.
“Good things come to those who wait… greater things come to those who get off their ass and do anything to make it happen.” - Unknown