We don't know how many addresses people have. But we do know that, excluding multi-sig addresses, an address belongs to one person or one organization.
No, we don't, not in the sense that we could say "there must be one person that (thinks that he) owns all the coins in this one address". The Mt Gox incident showed very clearly that a lot of people (far more than I imagined) are totally ready to give control of their coins to some other entity that typically will consolidate the coins to a small number of addresses.