So since we're talking about building stalls and filling them, how many horses would you think can be comfortably supported on about 5.5 acres of fenced pasture? All flat and grassy. No woods. Very pretty looking grass but I don't know what type. I expect to supplement with hay and grain so grass will not be their only source of nutrition... but I also would like to keep the grass looking/growing somewhat nicely and NOT turn this place into a picked over mudpit like some places end up Growing up in the mid atlantic I was always taught one horse per acre.
1 horse an acre is a good rule of thumb as long (in my opinion) as you have someplace for them to be other than on pasture for some portion of their day. Obviously you could have them stalled for 12 hours, turned out for 12 hours but in the spring, for example, you probably don't want them on the grass for 12 hours before it has had a chance to start growing.
So, a dry lot/sacrifice paddock or sacrifice pasture is really important to give your grass a chance to get established each spring (and a place to put horses during a week of heavy rain, or whatever). I also divide and rotate my grazing areas (using step in posts and solar electric charger) so that I can go behind and pick/drag/mow and let it rest a little.
Hard to say exactly how much is enough because in dry years (like last summer), my pasture died and I fed hay all year for the first time ever. But if it's a good growing season, 4 acres is more than enough for my 3 horses to have 12 hours of turnout/grazing (sometimes too much - they can get fat).