I have come to realize that the term is purely a production of emotion. And emotion can get the best of any logic. When you are with a friend for such a long time, and the bond is so tight, the person can be a “BFF”, as your feeling says. So to be fair, we should understand “BFF” as “a good friend that you like so much you can’t imagine losing them”. In that logic, there can be more than one BFF.