Most camping personnel will admit that the horses improve the first few weeks of camp, then they go along quite nicely, but by the end of the camping season tiny mishaps increase. It may be little things, a laid back ear, a glaring eye, a raised leg that warns, or a little buck. Sometimes it can be a real buck, or a race back to the barn. Sometimes it is marked by a real kick, a real buck, and a real race back to the barn, or a refusal to leave the barn at all. And it all happens right at the end of such a lovely summer.
Every horse has a limit.
Every single one. Some horses, however will shut down rather than risk inappropriate behavior. This is more a matter of intimidation than of the horse's great heart and not something to be proud of. A safer way to organize is from the other end. Take the number of horses available, the number of appropriate staff, the number of hours that each is expected to work and use those results to figure out how many riders can be accommodated.