Dunbar’s number

Dunbar’s Number is a favorite blunt diagnosis for the pains that affect rapidly growing teams. The number, which is somewhere between 100 and 250 describes a point at which a group of people can no longer effectively maintain social connections in their respective heads. What was simple from a communication perspective becomes costly. What was a familiar family that you saw wandering the hallway becomes Stranger Town.

~ Rands from, http://randsinrepose.com/archives/the-old-guard/

Also, Dunbar’s Number on Wikipedia.