When Dumbledore revealed the Horcruxes, Harry formed a mantra to remind himself which ones he had yet to find.
"The cup…the locket…the snake…something of Gryffindor's or Ravenclaw's…" was the way it went. The diary and ring were already destroyed at that point. Harry was sure Voldemort may have found an object from both founders.
I am pretty sure harry thought that there were six. the cup, the locket, the The Diary
Marvolo Gaunt's Ring
Salazar Slytherin's Locket
Helga Hufflepuff's Cup
Rowena Ravenclaw's Diadem
Nagini
But he didn't know that there was another. Up until after the diadem was destroyed, Harry thought there were 5 horcruxes- the diary, locket, cup, ring, and diadem. Then he had a vision in voldemort's mind and found out that nagini was one. Then he thought there were 6, until after the pensieve he talked to ron and hermione and discovered that he was one. Then he was positive that there were seven total. So when he didn't know about himself, he thought there were only 6. He didn't know about a 7th one.
posted over a year ago
Harry and Dumbledore had known there were 6 Horcruxes, Nagini being the last made. It was the fact that Harry was one too that Dumbledore hadn't told him.