Realising it has been months since I have even written a word, I found this bunny while searching for inspiration and decided to put it out there.
"Why would I make six Horcruxes?" asked Voldemort.
"Seven is a magically significant number," answered Harry as they circled each other warily, wands at the ready.
The Dark Lord smiled. It was a chilling sight.
"Very good, and yes, I once planned on making six containers for my soul, but then I thought 'if seven is significant, why not seven times seven?'"
Harry heard Hermione gasp.
"Forty nine?" Hermione whispered. "Impossible".
The Dark Lord let out a cruel bark of laughed.
"Not in the slightest," he said. "Only the first was even mildly difficult. By the time I reached forty nine, it no longer took any significant effort."
"No," said Harry, looking intently at his enemy. "You're lying, trying to trick us. I can see it in your eyes."
"True," admitted Voldemort easily. "After I reached that goal, I found myself asking, 'What is even more significant that seven times seven?'"
"Seven to the power of seven," whispered Hermione.
"Very good," said the Dark Lord with a look of annoyance at her for 'stealing his thunder'. "You may truly be as gifted as people say. Not that it will matter.
"So what say you now, Harry?" he continued, "Do you think you will be able to track down all of the pieces of my soul? Do you believe that even if you defeat me here, strike me down with some mighty magic the world has never seen, do think you will be able to find them all? No, I think not, and every day that passes will leave you weaker, wondering 'how long before he returns again? How long before he succeeds and kills me?'. You will never find peace Potter, and you will never find them all."
"He's right. They could be anywhere, anything!" said Ron.
Harry's mind was working furiously. They wouldn't be just 'anything'. He knew the Dark Lord well enough to know the man would have a plan, a grand ambitious plan that nobody would ever think of, that nobody else could be ambitious enough to even conceive.
Nobody except Harry.
"The Dark Mark," he said.
Voldemort's stopped smiling.
"That's why you brand all of your followers, isn't it? The Dark Mark is a Horcrux."
"Of course," said Hermuione excitedly. "That explains so much."
A/N There is nothing in canon saying a Horcrux was HALF of his soul or had any sort of limit on how many he could create - that's pure fanon. Why else would there Voldemort seem so intent on killing so many, where did so many inferi in the cave protecting the locket come from if not from Voldemort murders, and it also gives a bit of a reason why he would put an easily identifiable mark on his followers. It may even have also made them more than just 'loyal' (think Nagini).
At least, that was the idea behind this, but I just never found a place to use it.