The notion of the magical number of 7 (plus or minus 2) was devised by George Miller in 1956. (No, not that Mad-Max movie-producing-George Miller.) The Wikipedia page illuminates that the "7+/-2" concept is simply a hypothesis--not something for which you can find extensive scientific research.
That's ok--you won't find extensive scientific research for agile either, or for that matter, much of anything to do with software process. But we have stories! We have anecdotes! We have seen agile work, and we've also seen it fail. We've also noted that it doesn't "work" overnight: The whole point of agile is that you start somewhere, bite off a small chunk of work, and then continually reflect and adapt.
There are lots of elements in agile to learn and remember. Uncle Bob defines three laws for TDD. Kent Beck devised the four rules of simple design. The agile manifesto defines four values. And so on. We found at least a full deck's worth of such lists, and have started capturing and using these lists to prod ourselves when our memories fail. The goal of our "Agile In a Flash" project is to produce a web resource, a book, and a replenishable index card deck, all to use as tools in your day-to-day application of agile.
And we will go George Miller two better! Most of the lists fall into the "5 plus or minus two" category.
So who are we? My name is Jeff Langr; you can find more about me here. I've known Tim Ottinger for only a little over a year, so I'll let him speak for himself in his own blog entry. Tim and I are both Object Mentor alumni (and both of us contributed to Uncle Bob's Clean Code). Tim and I also don't see eye to eye on everything and we're both pretty picky, which is what I think will help make the Agile In a Flash cards the best possible set. Stay tuned.