STD etiquette

I know that with formal wedding invitations, each person in a household that is over 18 years old gets their own (i.e., 'Dr. and Mrs. Smith', then a separate one for say a 21 year old son or daughter)

Do I have to do the same when sending out STD's? Or is one per family adequate? I am trying to figure out how many I need to order :)

Thanks!