I thought the population density in Morrowind and Oblivion was a little low. I expected the capital of the Empire to be more...crowded. Maybe not Assassin's Creed style, but still, those big houses and those imposing streets seem to me rather deserted. The roads are pretty deserted too: no trade, no group activities, no ceremonials, no celebrations. Of course I do like that every NPC has a name, an occupation and many of them own a house. Since Oblivion they started moving, sleeping, eating and wandering pointlessly. This is likely to be pushed further in Skyrim with the professions and the (hopefully) improved Radiant a.i.
My question to you is how many NPCs would you like in Skyrim, knowing that the more they add, the less backstory and activity scripts each of them will have? We do know the map of Skyrim is about the same size as Oblivion, so let's take Cyrodiil as a reference. Do you want more "johndoed" population so as to allow some believable crowd activity (ceremonials, skirmishes, celebrations, hunting) or do you want every single individual to have a very detailed life, sacrificing the "crowd" factor?