Usually two horses will remain relatively calm even if the other horses are out of sight. Also a single horse is more likely to break loose. Then you are on foot. Almost any group of horses has at least one animal that wouldn't leave the rest under any condition. If time is short, this horse could be used as a wrangle horse and turned loose after the rest are tied for the night.
Bells help keep track of horses and make strayed horses much easier to find.
When turning horses loose to graze, bell all mares, loners or animals that have a tendency to stray. Bell straps should fit reasonably snug to prevent them from getting caught. Bells with a higher pitch seem to carry farther in the woods. If several horses are belled, bells of different pitches should be used. Weather conditions vary the distance that sound travels.