When considering the addition of wall sconces to your lighting plan it is a good idea to take a moment and consider the purpose. The function of the light source can dictate how much light is needed and how many lighting fixtures you want to choose. For instance, will the sconces be used to provide needed light in a dark hallway or to add some additional complementary light to a large room? Or, will the wall sconces be serving as a decorative accent?
Typically, wall sconces should be hung 60 inches from the floor and if the purpose is to provide functional light, then you would want approximately 6 feet between them. So for example, if you're adding wall sconces to a long hallway, you could need 3 or more sconces depending on the length of the hall.
Adding wall sconces as a complementary light source is ideal for a larger room like a living room or even a bedroom. You normally will have some type of ceiling light providing the primary light source and the wall sconces when placed strategically can brighten a dark corner or highlight a piece of art. When hanging wall sconces for decorative purposes - there are no rules, simply place them to highlight and add a warm welcoming light to your home.