you are in luck, because the best juicer to choose beets with are the less expensive Centrifugal types. I have a breville and liked it quite a bit, it has a very strong motor, and the chute is very large so you don't have to cut the produce up into small pieces and feed it into the juicer.
The juiceman juicer and Jack lallane juicers also work perfectly fine and both would juice beets perfectly fine.
As to how many beets you would need for x ounces of juice that is impossible to say because it would vary from beet to beet. It depends on how fresh produce is and the quality of the produce.