Given how plastic the brain is I suspect that the limit on social relationships is more logistical than physiological.<p>ie actually meeting, being social for long enough to develop a relationship and then maintaining that relationship with someone takes a certain amount of time.  Above a certain number of relationships the individual simply doesn't have the time or incentive to acquire more.<p>If there was a real incentive for someone to have more than 150 relationships I would expect that either a), their brain would rewire itself somewhat to handle the increased number or b) they'd make abstractions that would help them to maintain friends more efficiently.