There isn't really any word requirement and I don't think it gets denied too often... Mostly they just want you to have legit posts that carries on a conversation about a topic. I'd be a little skeptical about some of the "game" topics (eg "Count to a million posts", "3 word stories", "Which do you like better, A or B?"), especially if you just put in a minimalist answer. The "Which do you like better" topics are a little easier to expound more about why you like the one over the other, so you don't have to give just a one-word reply. Also, rattling off nonsense won't get your posts counted, even if it contains a lot of words. So the following wouldn't count:
Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Aenean commodo ligula eget dolor. Aenean massa. *bleep* sociis natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus. Donec quam felis, ultricies nec, pellentesque eu, pretium quis, sem. Nulla consequat massa quis enim. Donec pede justo, fringilla vel, aliquet nec, vulputate eget, arcu. In enim justo, rhoncus ut, imperdiet a, venenatis vitae, justo. Nullam dictum felis eu pede mollis pretium. Integer tincidunt. Cras dapibus. Vivamus elementum semper nisi. Aenean vulputate eleifend tellus. Aenean leo ligula, porttitor eu, consequat vitae, eleifend ac, enim. Aliquam lorem ante, dapibus in, viverra quis, feugiat a, tellus. Phasellus viverra nulla ut metus varius laoreet. Quisque rutrum. Aenean imperdiet. Etiam ultricies nisi vel augue. Curabitur ullamcorper ultricies nisi. Nam eget dui.
It usually isn't too difficult to get in your required number of posts. 30 posts in a month is about 1 post a day. So you can just find one topic that tickles your fancy, write in a reply that day, and your all set. But I know in practice, sometimes life will eat up your life... It's already over halfway through the month and I don't think I've even made 10 posts yet. I may just have to veg. some weekend on FC and make a bunch of posts in the forums, lol. But remember to try to keep up with this; the "10 cents a day" ($3 / 30 days = $0.10 per day) you make here plus the 15 cents a day for PTC, and 2 cents for e-mail can get you close to your cash-out in only 3 months.