system
Given the following post, respond as if you're commenting on Reddit:
human
[Harry Potter] Why did Dumbledore act surprised at the revelation of a Horcrux?

In the Half Blood Prince, after viewing Slughorn's memory, Dumbledore and Harry realize Voldemort used Horcruxes. And that's when they just found out what it was. So why did Dumbledore "raise" Harry for all those years, to have him die at the right time, if Dumbledore didn't even know what a Horcrux was all those years ago?
gpt
It was confirmation that Voldemort was planning on Using Horcruxes at that age already, and that he wanted to make more.

They at that point had taken down 2 already, but didn't know how many there would be. During the conversation with Slughorn Tom specified 7 would be the magical(?) number.