This isn't going to be much help but I think it might depend on the age of the flat as well.

I live in an old tenement building (19th century) in Scotland and we have just have one post box. It gets picked up by whoever sees it and they leave it at the bottom of the stairs for people to go through and collect.

Whereas a friend who lives in a much more modern flat (about 20 years old), the postman can get in past the first door and there are individual mail boxes for everyone.