Re: How much kanji do I need to know to read a light novel?
Well, to read all of it without assistance you'll need to go through several years of Japanese classes. So, "all of it" would be a good estimation. People who've done Japanese for years still need dictionaries for some stuff.
So ideally you would have to start with basic words, terms and sentences in romaji, then learning grammar, then learning some hiragana and katakana, eventually you'll get to learning kanji, and then continuing that pace for several years.
It's not quite as easy as knowing a few bits and pieces of kanji.