When I am driving and see a person holding 4, 5, or 6+ leashes, I am often concerned about those dogs. Though I imagine their owners envision them having a great time and this being a much better option than being cooped up at home, I'm not sure they are having a great time. Their body language often looks tense to me, with tight faces and stressed eyes. Why might that be?
Usually, their leashes are relatively short, 6' or less, and they are moving quickly, which means they can't sniff! That's the best part of the walk. (What's a walk with no sniffing like? Imagine you have google but can't search. You have a book but can't read. You have Netflix but can't select a show. You get the idea.)
They must go at one pace. Have you ever walked with a toddler or someone injured or just behind slower-paced people at a store? Did it feel odd to go that speed? Consider dogs - what if you are smaller or have shorter legs? What if you woke up a little stiff or sore? What if you have a headache today? What if it's hot and you don't tolerate heat well? What if you want to go faster?
What if one dog is ill or has fleas? In close quarters, they may share disease. (This is also a risk at daycare or the dog park.)
What if there's an emergency, like a loose dog charges up to the pack, or if a cyclist is going by and one dog wants to give chase?
What if one of the dogs is worried about other dogs or getting snarked at by another dog? The dog is being flooded with stress hormones.
How do they all get to one place? Are they all picked up, and then they walk? Is it possible to safely confine that many dogs in most vehicles? What if you are a dog who gets carsick? Do you have time to recover before walking?
Do dogs actually travel in packs if left to their own devices? No, not really; other than cooperative hunting, they tend to be somewhat solitary. You can google info about packs and street dogs for details if you want to learn more. Real packs are families of wolves, possibly wild dogs, and they are related as a family.
Can one shy dog benefit from walking with a more confident dog friend? Sure, but they should be friends first, and you would miss a lot of training opportunities for the shy dog if you were dividing your attention among lots of dogs.
How many dogs are too many? It depends, I suppose, and I would probably not feel comfortable with more than 3, and even then, only if they were well-matched, well-acquainted, and I was familiar with their behavioral quirks and not doing any active training.