All bearings are sold in packs of eight - 2 for each wheel.
How do you know which bearings are the best? Well, the simple answer is to buy Bones Reds bearings - more skateboarders ride these than any other bearing, and they are reasonably priced.
you may hear talk of bearings being rated on the ABEC scale - this is an industrial rating for all precision bearings used in machinery. if a bearing is ABEC rated, that means it is good enough to be used in precision machinery, so is certainly good enough for a skateboard. Generally speaking, a bearing with an ABEC rating of 5 or more will be very fast and smooth.