In the absence of Christian comment, perhaps I can play devil's advocate (pun intended)?
I think some of the above calculations assume that two demons cannot occupy the same space in our dimension :) - if this is possible then potentially the number is limitless.
Also, perhaps this is a reference to the number of demons which are able to control the person and that they only inhabit the body figuratively?
I suppose for a Christian, the question is 'does my soul inhabit my body or is it in some way separate but associated in such a way that it can exercise control?'. If it is the latter then I can see no theoretical limit to how many disembodied demons could exercise control over it either simultaneously - which I guess would make it difficult to do anything unless they act in concert - or one after another.
As far as the pigs are concerned, perhaps Jesus disconnected them from the human and attached them in some way to the pigs. The pigs being later destroyed would mean that the demons were no longer attached to the human, or anything else for that matter. Perhaps they simply re-assumed their former incorporeal existence. This would however raise the question of how they attached themselves to the person in the first place (which I don't think is addressed in the scripture in any detail) and what would stop them simply re-attaching themselves.