According to the memory that Harry extract from Slughorn, about Slughorn and the young Riddle discussing about Horcruxes, Riddle mentioned the magically lucky number SEVEN.
It might be very obvious, but at that time I am sure Slughorn wouldn't have thought (and he didn't) that someone would be willing to make one Horcrux, let alone a major seven as each require the caster to commit murder. The idea being that you lose a part of you with every act of killing and instead of losing that part, storing it in an object, as a means to prevent death.
Plus Slughorn's and Riddle's conversation wasn't general knowledge. It was difficult for Dumbledore to extract the true memory from Slughorn, and it was only managed by Harry when he used the memory of his mother Lily and Slughorn's guilt to his advantage. And why Slughorn go round discussing it?
So it is unlikely many (but maybe R. A. B.) would have thought that the number seven would be the key to Voldermort and his plans.
Possible and definete Horcruxes and notes:
1. Tom Riddle's diary - destroyed by Harry Potter.
2. Marvolo's ring - destroyed by Albus Dumbledore.
3. Mystery Horcrux, possibly Slytherin's (founder of Hogwarts) locket - destroyed by the rogue R. A. B. I say Slytherin's locket, because R. A. B. left a fake locket behind.
4. Helga Hupplepuff's (founder of Hogwart's) cup.
5. Nagini, who is Voldermort's snake companion.
6. The final part, of his soul, is within him.
The seventh part, according to Dumbledore was to be cast the night that Voldermort planned to kill Harry, when he was a child. Being that by killing his most likely killer (as pointed by the Prophecy), he would be able to create a hugely powerful Horcrux.
Other contender's of possible Horcurxes, disregarding the above theory, that is if Voldermort did create a Horcrux before attacking Harry, would be something belonging to Gryffindor or Ravenclaw - the other remaining founders of Hogwarts.
Gryffindor's only known remaining heirloom is his sword, which is in possession of the Headmaster or now, Headmistress of Hogwarts - but could Riddle have made it his Horcrux? And how?
So maybe, something belonging to Ravenclaw?
But how big is this number? Do you stick with Lucky Seven? Or create more? Personally, I think it could be no more than seven. Reason being that, Voldermort is a very determine person. He sets his goals and achieves them, this goals don't seem to change. He wanted revenge on his father, he got it. He wanted the locket and the cup, he got it. All that as a very young man. And I bet if he wanted to, in the 1st book and Harry's first year he could have given Harry the option of walking away - but no, he really really badly wants to kill Harry. Single-minded? Perhaps.
Plus, we never found out what kind of spell and magic it takes to cast a Horcrux, does it weaken the caster - hell, you are ripping your soul into tiny chunks - it has gotta hurt.
Anyone agree? Your thoughts...