Full disclosure: I rode all through my childhood and teens but haven't ridden regularly for a good 15 years or so and I'm quite out of touch!
Our neighbours have a small yard attached to their house which they rent out, and a field at the back of that which runs across the back of their property and ours. I'd guess there's under 2 acres of grazing, to my eye more like 1.5. The yard has stabling for 5 horses and the last tenants had 5 horses turned out most of the time. The field was grazed bare and basically just dry earth (or mud in winter). I don't doubt the horses were well fed; they had plenty of hay (even in spring!) and I guess hard feed too - they looked healthy but the land was ruined and heavy rain just ran straight off the top and into our garden which I'm convinced caused flooding last summer.
The last tenants moved out and some patchy grass has reappeared - now a new set of tenants has moved in with 4 horses this time! I'm shocked, I was always taught you needed 1.5 acres for the first horse and 1 acre per horse after that. Is that not relevant any more? Or is that only a rule for grass-fed natives maybe? I just can't understand why you'd cram 4 horses into such a small field!