I live in the city limits and have a fairly large backyard, but have neighbors within 100 feet on all sides of my house. I've actually found that the neighbors love coming by and looking and saying hi to my ducks when they're in the yard. They're the hit of the neighborhood! Even a marginally large backyard should be enough to keep some ducks. I wouldn't recommend more than 4 or 5, and I would probably keep 2 or 3 if I were you, but I think you're probably fine, esp. if it's legal.