Found this online image of the 'north end' mailboxes; I'd say 80 is a good estimated count for these metal mail repositories. And the NatGeo article that blog links to has the island's cottage count at 184:
http://ngm.nationalgeographic.com/ng.../fulltext.html
Some of us get island mail at the East Bear mail dock. And some have island mailboxes (with Bear Island addresses) on the mainland, such as at Cattle Landing.