I grew up in a suburban area. Our next door neighbors had horses on a half-acre of land, and they seemed fairly happy. If I were to take care of horses, how much land would I need for them to have a healthy life? Does it depend on the type or number of horses?
People's opinions on this will likely vary widely. Personally, I favour a set-up that allows enough pasture for horses to graze without need for supplementation in fair weather. I also feel strongly that it is better for the horse if they can be outside as much as possible. I know that many horses spend much of their lives in stables, but I have seen many negative consequences from this approach (both physical and psychological) and prefer to reserve stabling for harsh weather or other exceptional circumstances.
That being said, I would say the more space you can provide, the better. (It is nice to be able to have two or more paddocks available so that you can "rotate" in the spring when the grass is just starting to grow, etc.) But I would give ~1 acre per horse as a rough lower limit. Unless you are planning on keeping miniature horses or possibly small ponies or similar. In that case you might be able to get away with half an acre per horse, but I wouldn't really feel comfortable keeping any type of equine on less land than that.
Here in Germany the professionals learn a thumb rule stating that a horse of half a ton weight (which is a rather small standardbred) needs roughly 1-1.5 hectare per year for food supplement.
So when it comes to food, roughly multiply the number of horses by 1.25 hectare (which is 10,000 square metres). It depends on the ground, the soil, the plants, the moisture, weather and all, though. It also depends on whether you keep them outside all the time, or stall them up. The ideal for sure is using stables as little as possible (there are some cases where that might be appropriate, though). Most stables have far too little land; even most professional stables cannot afford the recommendations. So, a good thumb rule is: The more, the better.
Horses love to run and walk long distances, so if you have the choice of a long, narrow pasture, your horses might like that more than a square one, especially if they can get out of sight of their stables (I observed they do like getting some miles between, even though they'd never miss a meal and will come back for food ;D)
If you supplement hay, you can hold the horses in smaller spaces. Even then, the ideal would be above 100,000 square meters per herd, depending on the herd's size. They may also be happy with 1,000 meters per horse, though. When you supplement them, they will stop moving far distances, as the hay supplement usually has a stationary place. Feeding them solely from grass changes the movement situation a lot.
Oftimes what is best for the horses will be very costly in time and money for the humans.
Some factor that many forget is to look at how the horse grew up. If it grew up in a narrow stable, it might have problems adjusting to wide spaces. If it had the luck to grow up in an open stable with small pasture, it will love the open spaces, but it would still be happy with smaller areas. But if we are talking about a horse that is not used to fences, e.g. a mustang, then even the widest pastures might be too small. Take that on the example of stables. Horses that never lived in a stable will freak out after short amount of time when they are put in one. Horses that were born in stables are less likely to show this behaviour (though even then there are some that will freak out, can't blame them for that!).
At the moment, some stables have started building a path surrounding the pastures, which is available to the horses at all times, as a compromise between having not enough pasture to be able to allow horses to graze in winter, and wanting them to have some large arrow to move in. I grew fond of this system in places, where you just have not the possibility to even buy large enough meadows. I observed they will use these paths to play and race from time to time and like to use the possibility to have their time off when they need some time alone - yes, sometimes even horses want to be left alone, so best give them enough space that they can take their time off!
So, the final answer is: It totally depends. Best thing before taking care of horses yourself is to go explore the country and observe as many pastures and stables as possible. Watch the horses, analyse their behaviour, their needs and expression. If you want to take care of a bunch you will need to have good horse observation skills, anyway. Compare. A lot. Compare changes in behaviour that occurred after the horse was moved to some other place. This might sound time-costly, but it is the only way to get a good idea of one individual horses need.
Living in the largest city in Australia, most horses in our area, which is built up and in the suburbs, are given approximately 1/2 acre each. With this, they must be hand fed and exercised. The real estate comes at such a high premium that it's difficult to find agistments in some areas that will allow more than that. However there are usually ample avenues to take the horses for exercise (pony clubs, bush trails).
The further away from the city centre the larger the acreage per horse, where there will be the ideal, where horses do not need their feed supplemented from the property.
Having said that, I had 2-3 horses on a 7 acre property and they needed hay, as the grass type was not sufficient to feed them.