To "bake" a sweet potato in a microwave is simple to do but the timing depends on the size of the potato and the power of the microwave. For average measures on both these, try about 8 minutes on high. Let it sit for about 10 minutes afterwards, as it will continue to cook.
When using a conventional oven, the baking times do not change whether you are baking one sweet potato or a dozen sweet potatoes. The baking times only change when you put additional items into a microwave oven.