[QUOTE=ElementFarm;8223154]
what’s allowed is different from what’s workable which is different from what’s ideal.
Our neigborhood doesn’t have a #/acre, but no cloven-hooved animals are allowed (no goats, cows). I’ve seen and worked at establishements that had very minimal acerage and turnout (think 5x small paddocks that ranged from large round-pen sized to under an acre) and they maintaned 40+ horses (ranging from school ponies, to boarders, to several grand prix jumpers). It, sadly, was the norm for a horse farm in that area – land lots were small and property expensive (near/in urban areas). The horses had very limited turnout and there was no grass to speak of.
On the other hand, I’ve boarded at places with 10 horses on 20 acres. Sounds great till you have to bring in a hard-to-catch one…
I think it really depends on if you plan to have your horses live out 24/7 or be stalled part of the day. It depends on your set up (some grass paddocks, at least one sacrifice area) and how smart you are about rotating. Soil (clay vs sand for example) will play a part in how well the grass and pastures hold up.
I think with a well designed 3-acre farmette, you could comfortably have 3 horses. But layout, and maximizing all space, is vital.[/QUOTE]
Best answer by far, particularly the first line. That should be tatooed on inside of the eyelids of all horse owners. 
We get almost 60" of rain per year. That means lots of grass and mud. Around here cattle folks talk about “one cow, one calf, one acre.” Some do better. If I lived in the Big Bend country of Texas it would be “one cow, one calf, one section”*
While the Big Bend example may be a bit overstated if you’ve ever been there you’ll know why.
In answering this question the basis of that answer will be three elements: location, location, and location.
G.
*That’s a square mile, 640 acres.