I've never heard of regulations in OK more or less common sense dictates. I've got 4 horses, 3 share 4.5 acres and my boy is on 1 acre by himself but close enough he can keep company with the mares without getting to them. This to me is the limit, but I've known people who have tried to cram 10 horses in the same size area or smaller
It depends on the state/county and if you are going to feed hay or want them to get most of their summertime food from the pasture. Around here I know several people who keep horses on as little as 1/4-1/2 acre per horse, but it seems 1 acre per horse is pretty standard if you want decent pastures too.
In the area of MI that some of my family lives you have to own 5 acres to even have 1 horse, yet there is no rule that it all has to be fenced in for that horse. They just have to own that much.
Where we were looking out in Wyoming they were recommending 20 acres per horse because the area is not lush and while there is grass it is very sparse.
If you plan to feed hay & get no grazing from the pasture then you can have a pretty small area. It just needs to be large enough for them to be able to get some excersize - so room to safely trot/canter around.