Our farm is divided up into three group pastures, a quarentine stall, and three stud pens. The first group pasture is 3 acres and holds 9 miniature mares. It gets very muddy at times though.
The second group pasture is our full sized horses- its 14 acres and has our ten full sized mares and a filly. Its divided into three and we rotate it weekly. It isnt a dry lot.
Our other pasture is about 12 acres and has our mini geldings and full sized geldings in it. That's seven full sized geldings and ten minis. Part of it is sacrifice lot, part is pasture.
Stud pens (for one jack and two stud miniatures) are each 3/4 of an acre.
It works for us, but remember that one pasture is completely dirt and most of our horses are under 300 pounds. We also go through four round bales of hay a week. If you're wanting grazing land, theres no way that 2.5 acres is going to work. If you're willing to pay a lot for hay, go for it. If not, you need to really rethink this.