Well, if his hypothesis is validated this number should still apply to modern humans. You always have to remember that for 99% of our species existence we lived in groups of 150-250 individuals. Because biological evolution operates on very long time scales our bodies and brains are really still adapted to a hunter-gatherer lifestyle. This is also the case because natural selection does not operate on our species in the same way as it did throughout most of our evolution anymore (e.g., because most people live long enough to reproduce). Dunbar's number makes sense when you think about your own life. I know it does when I think about my own. How many relationships do I maintain (to any degree) at any one time? It certainly isn't more than 150. It takes up a lot of mental power to maintain social relationships. They are complicated and we only have so much space in our neocortex to handle that much information. It shouldn't surprise anyone that there is some limit on the number we can functionally maintain. That being said, how many practically do we need to maintain? Social relationships also take up a lot of time and energy. At some point we have to conduct a subconscious cost/benefit analysis when evaluating who is in our life and who isn't in our life. It would be practically impossible to maintain relationships with 1,000's of individuals. Dunbar's number has evoked a ton of controversy and interest. It is certainly an engaging idea that will require more research to solve.