This is a genuine Oster/Sunbeam replacement part.
Only one is needed in this unit.
Depending on how frequently the blender is used. The Sealing Ring can become brittle and cracked or compressed so that it will not create sufficient seal to keep the contents of the jar from leaking out of the bottom. If you use your blender often, it may be a good idea to order more than one at a time, so you won't have any down time with your blender.