Membership section shows your dedication and commitment to the specialty.
Some of them are free and others are paid membership. You do have to sign up with them.
You can simply search in google about different association. Take a look at the search sample below:
American Medical Association is a neutral one and everyone should use that.
Now lets say you are applying for family medicine then your membership should be related to family medicine. Don’t put multiple different specialty membership here. It might hurt your application.
So how many should you list here?
1 to 2 is perfectly normal.
If you apply for multiple specialty then you should focus on the one you really want and use their memberships.