Any reason(s) in particular that you’re certain that Horcrux-creation would have detrimental effects significant enough that Voldemort would create only one, maximum? I assume it must have some detrimental effects, because otherwise, given a Rational Dark Lord armed with a time-turner, we’d be looking at Horcruxes proliferating as fast as he can make them. Tens or hundreds or thousands of horcruxes, one on every muggle device launched into or out of orbit, if he could manage it.
I notice that you seem to take for granted the existence of “souls” in MoR, which is far from certain. Actually I would rate the possibility as decidedly uncertain, since if souls and their attendant afterlife existed, it’d put quite a dent in the entire motivation for Harry’s “conquer death and achieve immortality for everyone” program. And as prasannak noted, Harry has raised the alternate hypothesis that horcrux creation is less soul-fragmentation and more mind-uploading: “Maybe he found some way of duplicating the power of the Resurrection Stone, only he loaded it in advance with a complete copy of his brain state. Or something like that.” (Chapter 39)
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And I just remembered a brief exchange between Quirrell and Harry in chapter 46, which (to me, at least) hints at the existence of more than one horcrux. Quirrell asks Harry, hypothetically speaking, where he would choose to “lose something where no one would ever find it again.” (I assumed he was being all ironic again and was talking about horcruxes.) Harry reeled off a list of about 5 possible hiding places, to which Quirrell responds “All excellent suggestions… But tell me, Mr. Potter, why those exact five? …There is an interesting pattern to them.… One might say it sounds like something of a riddle.” (Riddle? Irony overload.)
It just dawned on me that those 5 hiding places could only be clues to a riddle (the riddle of where Voldemort chose to hide all his horcruxes) if they didn’t encompass the complete set of all horcrux hiding places, meaning that there are probably more than 5. Additionally, those 5 hiding places would only be good “clues” if they were themselves correct hiding places, from which further hiding places could be extrapolated based on some sort of shared similarity.
An idle bit of speculation, which has probably been brought up before, but it occurred to me that MoR Voldemort, being more intelligent than his canonical counterpart, may not have seen fit to stop at a mere 7 horcruxes. Why not simply make as many as (in)humanly possible, rather than adhering to some superstitious wishy-washy stuff about “7 is a powerfully magic number”? It is almost certain that the mechanics of horcrux-construction in MoR are different from those in canon (e.g. mind-upload rather than soul-splitting), so perhaps the limit that Canon!Voldemort faced (unstable soul-fragments) is not something that would be encountered in quite the same form as MoR!Voldemort.
To provide the merest scrap of substance to my speculation, I noticed that in Chapter 53 (TSPE, Part III), Quirrell states:
The bolded interests me, partly because of something Dumbledore states in Chapter 61 (TSPE, Part XI):
Slowly draining a victim’s life and being weak compared to his former power is certainly what occurred with Canon Voldemort possessing Canon Quirrell—perhaps MoR Quirrelmort benefits from a similar arrangement? If so, the adjective “starving” certainly fits.
Now, we already know that Quirrelmort likes to play ironic little games with what he says—in fact, he admits as much himself in chapter 51: The lips twitched again, and then went flat. “Then I shall skip it. Mr. Potter, you sometimes make a game of lying with truths, playing with words to conceal your meanings in plain sight. I, too, have been known to find that amusing.”
To return to the point, if hypothetically describing himself as “starving” falls into the same class of ironically veiled truth-statements Quirrelmort enjoys dropping from time to time (see ch. 49: “If only You-Know-Who had lived, you might have persuaded him to teach you some of the knowledge that would have been your heritage, from one Heir of Slytherin to another,” or perhaps “Tell them I ate [the dementor]”--->”I am a death-eater”, or again Harry had asked why Professor Quirrell couldn’t be the one to play the part of the Dark Lord, and Professor Quirrell had pointed out that there was no plausible reason for him to be possessed by the shade of He-Who-Must-Not-Be-Named), what might be the import of “nine-tenths dead”?
My thought was simply that “nine-tenths dead” might be a clue as to the number of Horcruxes MoR!Voldemort created.