In addition to Cry's post, you can also put in your word count a lot more time later, even after several years. You won't get credit or badges etc. for it but they'll have it show under your name on their site.

They even asked me about the books I have written that weren't on sale anywhere. So I put in a couple, what I didn't put in were the academic publications and a couple of translations, to which I had given away the publication rights anyway, and some of those, I doubt are available anywhere except in some organization or other.