British anthropologist and evolutionary psychologist Robin Dunbar developed the idea that based on our brain size, 150 people is the maximum number of meaningful connections any person can have. This is the number of people you can reasonably keep up with—you know enough about them to ask about their family or their new house.
Here’s another way to think of it: How many people could you run into somewhere and ask them something personal without feeling like you were being nosy?
ps - if you don’t want to read the whole Wikipedia article, maybe scroll down to the part about Gore-Tex under the ‘Popularization’ sub-head. Interesting stuff. Thanks again, Malcolm Gladwell.