Which vertebrae will be fused in the average scoliosis correction?

Depends. Often the thoracic and lumbar vertebrae are involved with scoliosis. The number of vertebrae included in the fusion will depend on the severity and location of the scoliosis. Ask your spine surgeon for your specific situation.
Usually. Usually all the ones involved in the curve. There are specific approaches, techniques & types of instrumentation that can help minimize the extent of the fusion & this can also depend on how stiff the curve is pre op on special imaging techniques but the whole concept of the surgery is to create a balanced spine from a front/back as well as side view or there could be other issues that develop.