In Harry Potter and the Half-Blood Prince, we learn that Voldemort has 7 Horcruxes. This number is surprisingly low, considering that
to get a Horcrux, you need to murder a person. Voldemort has killed thousands of people, so why does he have only seven Horcruxes? Did he have to cast a spell while killing them if he wanted a new Horcrux?
Are there some other requirements or limits to creating a Horcrux?
Well, there IS a spell to use...
From Half-blood prince:
“By an act of evil — the supreme act of evil. By committing murder. Killing rips the soul apart. The wizard intent upon creating a Horcrux would use the damage to his advantage: He would encase the torn portion —”
“Encase? But how — ?”
“There is a spell, do not ask me, I don’t know!” said Slughorn, shaking his head like an old elephant bothered by mosquitoes.
And... numberwise, Tom was already thinking of seven...
“Yes, sir,” said Riddle. “What I don’t understand, though — just out of curiosity — I mean, would one Horcrux be much use? Can you only split your soul once? Wouldn’t it be better, make you stronger, to have your soul in more pieces, I mean, for instance, isn’t seven the most powerfully magical number, wouldn’t seven — ?”
“Merlin’s beard, Tom!” yelped Slughorn. “Seven! Isn’t it bad enough to think of killing one person? And in any case … bad enough to divide the soul … but to rip it into seven pieces …
Then, later.. Harry talks to Dumbledore about it:
“But now, Harry, armed with this information, the crucial memory you have succeeded in procuring for us, we are closer to the secret of finishing Lord Voldemort than anyone has ever been before. You heard him, Harry: ‘Wouldn’t it be better, make you stronger, to have your soul in more pieces … isn’t seven the most powerfully magical number …’ Isn’t seven the most powerfully magical number. Yes, I think the idea of a seven-part soul would greatly appeal to Lord Voldemort.”
“He made seven Horcruxes?” said Harry, horror-struck, while several of the portraits on the walls made similar noises of shock and outrage. “But they could be anywhere in the world — hidden — buried or invisible —”
“I am glad to see you appreciate the magnitude of the problem,” said Dumbledore calmly “But firstly, no, Harry, not seven Horcruxes: six. The seventh part of his soul, however maimed, resides inside his regenerated body. That was the part of him that lived a spectral existence for so many years during his exile; without that, he has no self at all. That seventh piece of soul will be the last that anybody wishing to kill Voldemort must attack — the piece that lives in his body.”
So, he possibly COULD have made more (or perhaps not; his soul was apparently getting to be in pretty poor shape by this point) but his plan had been for seven, to make use of a magically powerful number of them. And remember, his was in such poor shape that attempting to make the last one ended up making an additional one that he didn't intend.. which suggests an inherent limit on how small of pieces the soul could be ripped into.
From the series, it seems as though Voldemort was the only person to have ever made more than 1 of them. And they had taken their toll. He became less human as he continued through the series, eventually losing his nose. It is possible that as he divided it even further, there might have been even more dramatic consequences, that he couldn't even fortell.
Also, 7 is a magical number, which is why he wanted to divide his soul into 7 parts.
Murder alone isn't sufficient to create a Horcrux, there is some kind of further enchantment to create a horcrux. What it is is a well known secret, and is never mentioned in any media, according to the Harry Potter Wiki.
I think a quote from an answer to a different question might also add something here:
JKR: I see it as a series of things you would have to do. So you would have to perform a spell. But you would also - I don't even know if I want to say it out loud, I know that sounds funny. But I did really think it through. There are two things that I think are too horrible, actually, to go into detail about. One of them is how Pettigrew brought Voldemort back into a rudimentary body. 'Cause I told my editor what I thought happened there, and she looked as though she was gonna vomit. And then - and the other thing is, how you make a Horcrux. And I don't even like - I don't know. Will it be in the Encyclopedia? I don't know if I can bring myself to, ummm ... I don't know.
This quote is from an Interview with Jo Rowling which was originally brought to my attention by Richard when he was answering the linked question.
My point here is, obviously, the process of making a Horcrux is not just more than murder (and murder is obviously to be differentiated from killing, as well), it is magnificently horrible.
Now obviously Tom Riddle wasn't afraid of getting his hands dirty, but one gathers that making a Horcrux is an involved process and the fact that he killed many on his rise to power is only one part of the puzzle.
It is also worth remembering that he needs things to make into Horcruxes:
'And they could be anything?' said Harry. 'They could be old tin cans, or, I dunno, empty potion bottles ...?'
'You are thinking of Portkeys, Harry, which must be ordinary objects, easy to overlook. But Lord Voldemort use tin cans or old potion bottles to guard his own precious soul?'
Harry Potter and the Half-Blood Prince - p.471 - Bloomsbury - Chapter 33, Horcruxes
On the whole, though, these are minor additional points. I think the main points are his fixation on the number 7 and the danger of having more than one Horcrux, alluded to elsewhere.
'And the more I've read about them,' said Hermione, 'the more horrible they seem, and the less I can believe that he actually made six. It warns in this book how unstable you make the rest of your soul by ripping it, and that's just by making one Horcrux!'
Harry Potter and the Deathly Hallows - p.89 - Bloomsbury - Chapter 6, The Ghoul in Pyjamas
Let's not forget, the whole reason he had the tête-à-tête with Slughorn was because he wanted an opinion on what would happen if he went for the seven-part soul.
As an addendum to the existing answers, he may have had multiple parts of his soul within his body though the accidental creation of Harry as a horcrux means some explanation of how he has kept those pieces within him is necessary.
