any more than that and information is traveling more than 6 hours back. (at least from the perspectives of the earliest and latest self-clone)
I believe the only restriction is on not traveling back more than six hours by wall-clock time. It's never stated that you can't travel back into the same hour more than six time using more than one Time-Turner.
The universe makes it rather obvious that you can't. How do we know that? Because the economics of Time-Turners is such that they are only valuable if you have exactly one and any additional time-turners are irrelevant. If time-turners worked that way then...
... You would want as many as you could get. And Hogwarts wouldn't be able to loan them out. If each person can only use 1 time-turner (as I say), then the economic demand is at most the population who's aware of time-turners. If you can use infinitely many time turners, then demand is without li...
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: