An increasing body of research is suggesting the humans have evolved for a group size of about 150.   Known as “Dunbar’s Number”, the idea is that in groups larger than this size our efficiency breaks down.    I think the working assumption is that we cannot track more than this number of people without losing a lot of resolution, and that we work best when we have a good and high resolution relationship with people:  http://www.commonsenseadvice.com/human_cortex_dunbar.html