Ball bearing and ball bearing rails

It depends on the level of detail you want. If you are using the BBs in an assembly, you'd probably just model the outline of the assembled bearing as a single part. If you are a ball bearing designer, you'll model each component of the ball bearing with sufficient detail to manufacture it.

Peter -  You will need to constrain the balls the same way as the roller chain - in other words you need to use origins and planes to orientate the balls