One horse is lonely, two are buddies and three or more is a herd. When turning out a herd of horses for exercise and grazing you want to provide for them a safe and nutritional environment.
The more horses that are turned out together the more likely that conflicts will arise that could lead to injuries. Some barns opt to separate mares from geldings to avoid relationship problems. Others like to turn out older, sedate horses together while younger ones frolic with their rambunctious peers. If space permits, some stables pair up equines in smaller paddocks instead of one big pasture. Small turn out areas are also good to introduce a new member to the herd next door.
No matter the designated system of herd management, the priority is that the area is safe. Check the gate area, pasture ground, and fencing as you lead horses in and out. Make sure nothing is broken such as fence boards and that glass or plastic bags are not littering the area. Be sure that gate is secure when shut. Horses can weaken gates while rubbing or pawing in anticipation of being brought in at feeding time.
No matter the season, shelter is needed for those turned out more than a few hours. Of course, clean water should always be available too. The more horses there are to be turned out, the more space will be needed. Ideally allow one to two acres per horse.
If the pasture is for nutrition then rotating will be beneficial for the soil as well as the health of the herd. Parasite exposure will be reduced if horses are rotated with other livestock since parasites are species specific. Or allow a pasture to be empty for a few months with the manure piles broken apart to kill the larvae. Small paddocks that are used regularly should have manure removed every couple of days. Mowing will aid in weed control and fertilizing an empty field will promote healthier grazing when horses return to it.
Sometimes hay has to supplement or replace the grass especially in the colder months. When feeding, break apart a bale into separate piles several yards apart with one or two piles more than the number of horses to avoid fights.
Remember that your goal is for your horses to have a safe exercise area and nutritional grazing pasture. Pasture herd management will take planning and hard work but will benefit you and your horses in the long run.