I used to ride with a trainer who had immigrated to Canada from Switzerland. Initially he was appalled at the number of classes that horses were doing until he actually went to some shows and realized it wasn’t as taxing on the horses compared to what he was imagining.
A huge part of how much a horse works at a show is dependent on their prep/warm up. Both of my geldings have been there, done that. We usually walk to the ring, flat for 10-15 minutes to warm up, trot a crossrail, jump a vertical and an oxer off each rein, go into the ring and show back to back hunter trips. The undersaddle might last 10 minutes, usually less. One gelding isn’t a great mover so we don’t always do the u/s.
At most I’m on the horse for an hour, usually it’s more like 30 minutes. They typically don’t even break a sweat. It’s less work than they do at home. Fewer jumps than an average lesson.
The green beans may require some schooling classes, a longer warm up, or a lunge in the morning to get the bucks out.
Definitely fewer classes/day in the jumper ring. Trips are rarely, if ever, back to back, which requires additional warm ups. Courses are longer. If it isn’t an immediate jump off than that’s another warm up. The speed, twists and turns are also more strenuous than a simple hunter course. Our trainer usually lets jumpers do 2-3 classes/day over a weekend show, or 1-2 classes per day for a week long show when the jumps are under 1.10m. Anything higher and they’re usually only doing 1 class/day or less.
There will always be exceptions, a bad horseman is a bad horseman (or woman). If they’ve consistently got horses showing 5+ trips/day for weeks on end, then there’s probably a whack of other issues as well.
Any trainer that I’ve worked for or ridden with is careful about the number of classes per day, and isn’t afraid to scratch if needed. They typically try to save their good horses as they get older, ie just showing in special classes or at their favourite facilities.
Current trainer actually kicked someone out last year because the owner was always pushing to do more and more classes. Trainer didn’t want to be associated with them, or the inevitable break down.
One facility in Ontario hosts A shows and over the last few years has started hosting a few B level shows as well. Sometimes they have everyone pay a flat rate, but they limit the number of classes that you can enter.