Does BC care if you have 40+ family members who have graduated or is it just parents? Would it help you at all?
(my parents did not go however both grandfathers are triple eagles and a family member is a teacher)
Legacy at BC is parents only per the website.
http://www.bc.edu/admission/undergrad/process/freshman/faq.html
would there be anywhere to put that you had multiple other family members who went or do they just not care?
They would not care and it would make it look like you are grasping at straws.
Unless your family members know people at BC, nothing besides parent gives you an advantage in admissions decisions.