People often ask how XP scales. One hundred people can't plan their work in detail in a single meeting once a week. One hundred people can work together in a spirit of communication, feedback, simplicity, courage, and respect. Creating and maintaining a community of one hundred is a much different job than creating and maintaining a community of twelve, but it is done all the time.
The number of people on a project is not the only measure of scale for software development. Software development scales along many dimensions:
Number of people
Investment
Size of the entire organization
Time
Problem complexity
Solution complexity
Consequence of failure