Quite a while back Jacob Nielsen published on his site an article suggesting that 5 users are enough to uncover most of the problems in an interface when user testing. There has actually been quite a lot of research on this subject. Over at the measuringusability.com site by Jeff Sauro you can can read A Brief History of the the Magic Number 5 in usability testing to find a pretty well laid out chronology on the research.
UX and Agile
A topic that seems to be occurring with more regularity is whether UX can work with Agile methodologies. I think many UX folks may have struggled with organizations that are planning or already use an agile development method for a couple of reasons. However, lets start with understanding what Agile is and what it is really intended to solve. In my humble opinion, Agile is actually designed to address the fact that there is little or no UX process or UX people in the places that pioneered and use this approach.