Ideal XP Team Size
Because it is not really feasible to use larger teams in XP, teams have to find a way to delivery more value in the available time. The best way to do this is to follow the advice given by Jim Highsmith: “Start Earlier.” By avoiding wasting month and months during the early inception phases of a project, XP projects effectively start earlier and hence have more time to delivery essential functionality.
I would suggest that XP is best suited to projects with a range of 4 to 12 programmers. The good news is that a great many projects fall in into this range. Organizations are very wary of big projects, so most projects are on the small side. With software development shifting to object-oriented languages, the functionality available in the class libraries has made it easier for small teams to delivery bigger systems. Small teams of developers who really know the class libraries well can be extraordinarily productive by leveraging the power of the existing libraries.
— Pete McBreen, Questioning Extreme Programming, Chapter 13