In a memory of Dumbledore's, Tom Riddle asks Slughorn about Horcruxes. He says that he wants to split his soul into seven pieces. But Harry is the seventh and unintended one. So aren't there 8 Horcruxes in total, considering that Voldemort already split his soul into 7 pieces? Or have I made a mistake while counting?
Harry isn't a Horcurx. JK Rowling has specifically stated this.
So that's why your count is off. Voldemort intended to split his soul into seven pieces (seven being a number which he believed to have special, magical properties) and make six Horcruxes out of them, keeping one piece inside his body.
But when the Avada Kedavra backfired on him, that split another piece of his soul, which got embedded in Harry's scar, unbeknownst to Voldmort.
Because he didn't know his soul had split, he did it once again to make his sixth Horcrux. So with six pieces of his soul in Horcruxes, one piece inside his body, and one piece inside Harry's scar, that's eight pieces of soul, but six Horcruxes.
- Horcurxed in Tom Riddle's diary
- Horcurxed in Peverell ring
- Horcurxed in Hufflepuff cup
- Horcurxed in Slytherin locket
- Horcurxed in Ravenclaw diadem
- Embedded in Harry's scar
- Horcruxed in Nagini
- Remained in Voldemort
You were the seventh Horcrux, Harry, the Horcrux he never meant to make.
Albus Dumbledore - Deathly Hallows - Page 709 - US Hardcover
Turns out Dumbledore was wrong.