It's not a random number, as you possibly may have guessed. It is based upon Dunbar's number or what is sometimes called the '150 rule'.
It is an approximate number of the limit of people one can have "stable social relations" with. Malcom Gladwell wrote about it as well in his book, The Tipping Point.
Groups larger than 150 begin to need different rules of organization and laws to remain cohesive.
It's mildly noteworthy as well that the 150 rule also comes up in Stephen Wolfram's cellular automaton. Maybe much like fractals play a larger than understood role in the development of life, so does the '150' rule.
On a quiet week on the blogging front, I thought you might like to know where the 150 number came from!
This piece from Mind Sport is excellent at explaining why the 150 rule is important to organizational size, http://mindsportlive.com/Articles/Article/?articleId=254