I think it depends on whom you ask, but I did this last year. I had five chapters already written. So, I started my word count from Chapter 6 and went from there.

Remember that the whole point of NaNo is to get you to complete a novel. You aren't really competing with anyone but yourself - either you win (meaning, you write 50,000 words in a month), or you don't. In my opinion, if you start on Chapter 6 and write ANOTHER 50,000 words toward a novel project that you've been stuck on, it's still a HUGE WIN.