That's just the usual limit on information not traveling more than six wall-clock hours back in time, total. It doesn't say or imply that you can't loop yourself more than six times within a small stretch of wall-time.
Actually, if you can loop yourself more than six times at any small stretch of wall-time then you can get more than 30 subjective hours in one 24 wall-time day.
But it's implied you can't actually do that, which is why I think no more than 6 copies at any given time. Plus, if it were possible you could basically use any one day as a stopping point groundhog-day style in which you can (for example) brute-force read the entire Hogwarts library.
At any rate, the general limiting principle is that information cannot travel more than 6 hours backwards, Which I th...
EDIT: New discussion thread here.
This is a new thread to discuss Eliezer Yudkowsky's Harry Potter and the Methods of Rationality and anything related to it. With two chapters recently the previous thread has very quickly reached 500 comments. The latest chapter as of 17th March 2012 is Ch. 79.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author's Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven, eight, nine, ten.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: