I agree that 40 acres for 40 horses would not be enough ground.
Avna has pointed out the dangers of large (unnatural) herds. Rarely would you get that many horses together in the wild.
Horses are notoriously bad grazers, land they are on needs to be managed or you will get large swaths of long sour uneaten grass.
Yes, horses do need turn out, but as someone who has had majority of horses stabled majority of their day, I can tell you that in driving rain and wind, if I turned those horses out those horses would go out, first thing they did was roll, have a gallop around the field, roll again and return to the gate wanting back in.
The youngsters we bred were housed in the winter at night, in a large barn together. Open the gate to let them out in the wet and they would go out, turn around and walk back in.
In the summer they would, when the flies were bad, want inside. When brought in they would all be lying down snoring away within half and hour.
Forty horses is a lot to keep and have them pay their way. I know, several years ago admittedly, that a horse needed to 'work' three hours a day, six days a week, to cover costs. Feet/veterinary/ feed/utilities on buildings and wages of staff.
The idea of having a large herd running free sounds great but practically would be a nightmare for me!