This is a genuine OEM replacement part, it is sold individually.
By pressing the bulb, It will purge all of the air out of the carburetor.
Then fuel will come up from the fuel tank through the fuel line then into the carburetor.
Then the excess fuel will be forced back into the fuel tank through the fuel return line.
At some time the primer will lose its elasticity or crack and need to be replaced.
We have an article and video on How to Replace a Primer Bulb, in our Repair Center. That can be found on the front page of our site.