Why did tom riddle ask how to make 7 horcruxs years before he accidentally made harry into one?

Well Tom Riddleasks professor Snobbelvom, to find out if it is possible to make more then one (as seven). Heasks because in all of the books he have read, there had only said something about one, because it would be to terrible for a person to make more then one, they thought. But Tom Riddle knew that making seven would be smarter.

And that's why he asks.

The reason that he ends with making Harry Potter into one of them, is that he keeps a memory from his past in each of them. And even tough, that might not be a good memory (losing his powers) but it's still a big part of his past.

And that's why Harry is one of the horcruxes.

Another answer:

Tom Riddle wants to learn a lot about magic and defeat death.