Generally, you would use more than one. The tractor should be matched to the power needs of the implement you are pulling. Generally the biggest power need is for the plow, and a 5 or 6 (or more) bottom plow will need a larger tractor than a 3 bottom plow. Then you will have a disk or roller harrow, which can take a smaller tractor again depending on its width. On top of that, you may want a pretty small tractor for pulling a rake or other lightweight jobs. A 4 wheel drive tractor gives you more ability to apply the tractor's power to the ground without slippage, but a 2WD tractor can do just as well with duals on it. Then again, as you move up in horsepower, you need power to more wheels to avoid excessive slippage.

All that said, a tractor of around 100 hp is probably able to handle most of the equipment needs without being excessively over or under sized, with maybe the exception of plowing where you would need to use a smaller plow than is standard these days.

Jump on the Calif. Ag Extension website and search for crop budgets as these will generally give you a pretty good list of common equipment. Many states will have their own budgets through the Ag Extension listings at the land grant university.