Robin Dunbar is an anthropologist and he really did come up with Dunbar’s number.
Dunbar's number is a suggested cognitive limit to the number of people with whom one can maintain stable social…en.wikipedia.org
But it’s history only in the future of my story :) The theory behind the number got developed in the nineties. You would probably not have gotten this in school, I doubt it got into the standard curriculum these days either.
But you are right, everything comes from somewhere — and this sort of thing, human relationships and the dynamics thereof, was something I started doing research on as my marriage was failing. It was part of my attempt to make sense of my situation and find some sort of thing to hold on to as my world crumbled around me. Dunbar’s number stuck. I’ve been thinking about how we could build better societies based on this line of thinking for about 5 years.
As you can see, I found disadvantages to the idea as well.