I've been doing a QnA this month while I work on my next chapter. The impression I get from my experience is that you can probably count on somewhere betwen 25-50% of your regular commenters to ask a question for it. So I'd imagine with 1K subs you'd get plenty of questions.
I had around 70 subs I think when I first announced it, I put it in an image at the end of the updates (since I don't think mobile users see wall posts, and not everyone reads the author notes haha). I didn't get a ton of questions but I got some really good ones that were interesting and fun to illustrate, so I'd call it a success!
I've got one more going up next week, but here's the three I've done so far if you were wanting to look at examples.
Good luck with your QnA! 