I don't remember any numbering scheme; Dumbledore had some suspicions about what the remaining horcruxes might be, but it isn't clear which one would be "#6". I believe the last horcrux suspect he mentioned was the snake, Nagini. That might have been it.
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Lord Volemorts horcruxes included: - Tom Riddle's Diary - Slytherin's locket - The ring - Helga Hufflepuffs cup - The diadem of Rowena Ravenclaw - Harry Potter - Nagini the snake
Yes, Dumbledore is killed by Snape in Harry Potter and the Half-Blood Prince.
Voldemort made seven Horcruxes. Harry destroyed one in his second year and Dumbledore destroyed one just before his sixth year which left five Horcruxes to destroy (although he only thought there was four). In the end, every Horcrux was destroyed by different people (Harry, Dumbledore, Ron, Hermione, Crabbe, Voldemort, Neville)
This is a very broad question, and not worded correctly. Severus Snape killed Albus Dumbledore in the end of the sixth book. Dumbledore had asked Snape to kill him, to save Draco Malfoy's soul because of the Unbreakable Vow that Narcissa Malfoy made Snape take in the beginning of the sixth book since Voldemort had ordered Draco to kill Dumbledore.
Albus Percival Wulfric Brian Dumbledore dies in Harry Potter and the Half Blood Prince, the sixth book in the series, after being murdered by Professor Severus Snape on the highest Astronomy Tower.