150 is Dunbar's number, made by extrapolating a monkey species tribe size / brain size correlation to human brain size. Something interesting to think about, but not exactly hard science.
The book Loonshots says as much. Extrapolating from monkey brain sizes to human social interactions is a bit reminiscent of phrenology IMO, but it is an interesting observation. Anecdotally, I've found it to hold true, particularly in startup land when companies transition into blitzscaling hypergrowth.
At Meta, we collected tons of real world data to pretty definitively prove this is true. Humans can only really relate to about that many others a time. And most people have about 5 others that they are actually close with.
