conuly

Hmm, now I have to get the book out and check... OK, here are two quotes from Slughorn in chapter 23 (on pages 464 and 465 in my copy of the book):

'Well, you split your soul, you see,' said Slughorn, 'and hide part of it in an object outside the body. Then, even if one's body is attacked or destroyed, one cannot die, for part of the soul remains earthbound and undamaged...'

'...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 -'

I'm pretty sure that means that each time a person kills someone some of their soul gets torn off, they can then hide the ripped off part in an object to form the Horcrux. Here's another quote from the same chapter, this one is Riddle talking about about the number of pieces he wants his soul in:

"...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 -?'

I'd take that to mean that he wants his soul in seven pieces, not ripped seven times, and Slughorn interprets it the same way. Oh, and then a few pages later Dumbledore corrects Harry after Harry says 'He made seven Horcruxes?':

'...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 existance 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.'

Dumbledore could be wrong, but assuming he isn't there are six Horcruxes and seven pieces of soul.