A team of 20 developers is a large team.
Although XP can probably scale to large-size problems [http://www.thoughtworks.com/library/XP
On A Large Project-A Developer's View.pdf], it was never intended to be used with large teams. Kent Beck [Beck, 2000, p. 157] suggested that even a team of 20 programmers would be a stretch. A realistic team size limit for XP is probably 12 programmers. So the real question is, how big a problem can a team of that size handle?
Although the traditional approach to software development has involved throwing as many bodies at a problem as possible, maybe there is another way. Small teams working in an environment that is conducive to learning can achieve extraordinary ...