As a dog walker, is it normal to walk clients’ dogs off leash?

I always thought dog walkers walk their clients’ dogs on leash unless in a secure and enclosed space, like a private dog field. Is normal to let them off leash? A local dog walker has been walking 8/9 dogs off leash and two run away last week. Unfortunately, one died. I don’t know how one person can possibly control that many dogs off leash.