I wouldn't worry too much about word count when writing the book. Word count is for editing. If you're writing a YA book, you want to get it somewhere between 60-80k. But then again, there are no rules that cannot be broken. There are some that are hovering around 50 and some at 90-100k. So, just write the best book that you can. Revise and edit well and see where you end up. A well-written book is a well-written book.