How many friends can a average person keep ?

150, according to Dunbar, an anthropologist at the University College of London. Dunbar wrote a paper on Co-Evolution Of Neocortex Size, Group Size And Language In Humans where he theorized:
... there is a cognitive limit to the number of individuals with whom any one person can maintain stable relationships, that this limit is a direct function of relative neocortex size, and that this in turn limits group size ... the limit imposed by neocortical processing capacity is simply on the number of individuals with whom a stable inter-personal relationship can be maintained.

This number of 150 has become "Dunbar's Number" and has been taken as a mean size for many kinds of effctive social groups. This number was also cited by several recent books, including Malcolm Gladwell's best seller The Tipping Point.