@Ciel It depends on the artist. My artist only stenciled a little bit of my outlines and freehanded the rest. The artist will probably have the stencil ready and you probably won't have to pay for the time he's spent drawing it up. If the artist is freehanding the outlines he'll probably charge you for it.
As far as stenciling for each session, that depends on a lot of factors, such as: size, subject matter, amount of detail, how long you're able to sit, etc. You might only need to stencil for the first session, if you want a more simple and smaller back piece. If you want a traditional Japanese back piece, it will span for your neck to the back of your knees, and you might have to stencil for multiple sessions.
Hope this helps you.