"A theoretical cognitive limit, based on the size of the neocortex, to the number of people with whom one can maintain stable social relationships. These are relationships in which an individual knows who each person is, and how each person relates to every other person."
The Wikipedia article adds: "No precise value has been proposed for Dunbar's number, but a commonly cited approximation is 150."
I heard about it here: http://docs.google.com/Present?docid=dd729jdt_229hhhz67dg
This takes me towards me question of the limits on how many people one person - any person - can really know. A stark accounting of this number is quite sobering... the most obvious conclusion being that no-one really knows what is going on except imperfectly, indirectly, and in an extremely limited way. No wonder so many people want to believe in a god!