Is there a policy on that? Or could my dog be walking with a bunch of other dogs?
It really all depends on the walker, some will walk many dogs at once, some will not so make sure to discuss this with your walker during the meet and greet. If your dog is not the best at walking nicely on a loose lead or in a heal, if your dog has any little quirks or things they react poorly to, things like that, I would ask for my walker to walk my dog on his own, at least until the walker has shown me that he understands my dog's specific needs and can handle them even if he is walking other dogs at the same time.
I personally will not walk more than 3 medium to large sized dogs or 4-5 small ones at once, I just do not find it safe or responsible. It can be physically too demanding, not all dogs have the same level of obedience, if something happens with one of the dogs, having a whole bunch of other dogs make it very difficult to deal with the one dog having an issue, not all dogs walk at the same speed or have the same level of physical fitness, some do not do as well as others in extreme weather, etc, some dogs can only be walked by themselves. I like to personalize the experience as much as I can and adapt the walk or activity to each dog so I do not like having too many dogs at once. Plus I would not feel right charging a client full price if their dog is only getting a fraction of my attention.
Also, many cities have by-laws regarding the maximum amount of dogs one person can legally walk at once, licensed professional dog walkers included, so I would definitively not hire a dog walker who is not aware of such rules or willing to break these rules with my dog or anyone else's for that matter.