The number hasn't increased as it used to be 7± 2. Now it is 6 ± 3. So the max is still 9, but the range was increased in the Scrum Guide as there are plenty of successful Scrum development teams with 3 or 4 members.
The original 7 ± 2 came from a psychology paper called "The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information" where it demonstrates that there are limits to how much information we can keep in our heads. In Scrum teams, as in other teams that work in complex environments (Marines, Navy Seals, Sports), keeping high bandwidth communication and relationships is essential. This becomes more difficult once we pass that magical number of '9'.
More information about that paper here: http://en.wikipedia.org/wiki/The_Ma..._Minus_Two
Hope that helps!