A. We have over 600 muscles in the human body. Think about that for a second! Over 600 muscles working together to support, move and shape our body! Each muscle has a set number of fibers which is determined by genetics. You cannot increase or decrease the amount of fibers. So you cannot "Get more muscles." The shape of your muscles are genetically determined as well. Some people have naturally long muscles with short tendons while others have shorter muscles with longer tendons. You cannot change the shape of your muscles.
On a positive note though, YES you can change the SIZE of the muscle fibers. While you can't "Get more muscles", you can make the fibers larger and thus fuller looking. Muscle fibers respond to stimulus. By participating in a strength training program you can cause muscular hypertrophy or growth. If we are sedentary, muscles atrophy or shrink. This not only affects our ability to perform daily tasks but affects the shape of our body. For instance, many people complain their butt have become either flat or wider as they age. This is because the muscle fibers have atrophied. They become smaller which affects the shape. This is also bad because with atrophy of the muscles, the metabolism slows, which means more body fat stored... yup on the butt as well as other places!
With a well structured strength training program, you can increase the size of your muscle fibers, which changes the body's shape and size overall. If you're looking to get rid of the flat butt, or tone certain parts of your body, build up the muscle in that specific area. The muscle bellies will look firmer and fuller.