Dunbar’s number is a suggested cognitive limit to the number of people with whom one can maintain stable social relationships.
What is Dunbar’s magic number? It’s smaller than you think, in an era when people compete for 1000s of Facebook “friends.” (Are they “friends” if I can have 1000 of them?)
Not for mammals, anyway. It’s a brain thang: Dunbar’s number is called a “cognitive limit” because British anthropologist Robin Dunbar
theorized that “this limit is a direct function of relative neocortex size….the limit imposed by neocortical processing capacity is simply on the number of individuals with whom a stable inter-personal relationship can be maintained.”
On the other hand, by “stable social relationships” we don’t mean just the number of people you can fit in your living room, we mean 150 people as a working number:
These are relationships in which an individual knows who each person is, and how each person relates to every other person…It has been proposed to lie between 100 and 230, with a commonly used value of 150.
This is the village it takes to raise your child–your project, your fundraising, your new business. This village is where you base your search for support for your work. This is Home.
Why do it this way? Because it’s easier this way:
numbers larger than  generally require more restrictive rules, laws, and enforced norms to maintain a stable, cohesive group.*
The next time you wonder where to begin, focus on your core 150 friends, your “true fans” and allies, your friends, and those others who know you best. They’re your “community.” Start there.
*This post is written with thanks to Writer’s Digest’s June 2012 issue about what numbers of fans it really (only) takes for a writer to become successful. All quotes about Dunbar’s number are thanks to and from the Wikipedia entry on Dunbar’s number.