The challenge of NaNoWriMo is about 50000 words in one month - brand new. Technically they don't care how many books that is. You're allowed to write as many words as you want or as you can during November and count them toward your goal.

Novels have their own rules about how long something ought to be, however. It depends on your audience, your market, and your genre. Most mainstream, contemporary novels are somewhere between 70 and 90k, especially for a debut author. Established authors can get away with larger works. Speculative fiction (scifi, fantasy, and their cousins) can often get away with up to 120k because of the world-building aspect. Younger audiences tend to have shorter works - think YA around 40-80k and MG 20-50k.

While these rules aren't set in stone (and I'd have to look somewhere to find another reference), you'll notice that is different than NaNo's 50k goal. There's nothing wrong with that. Some books are that short. 50k is about 250 pages, so you can compare that to books on your own shelves and in the library or bookstore.