NO. NO. NO. NO.
Telling people they will get a reward for posting on a forum will not get them to engage in the discussion. It will make them spam like there's no tomorrow. If you tell people they need to get X number of thing, they will find the least-effort way of getting it, and that is the complete opposite of what forum posts should be about.
People should get involved in the community because they want to, because they like it, not because it gives them some virtual item they need for completeness' sake/to feel superior about. "Get X number of thing", or any quantifiable metric is a completely asinine way to measure community involvement. Community involvement isn't about doing a lot of stuff, it's about doing stuff that matters. It's about helping people, it's about engaging with people, it's about making people feel welcome, it's about giving people something to think about, it's about bringing people together.
Community involvement is personal, and any reward for it should be likewise. As soon as you have an automated system handing out rewards for reaching some set target, you're not rewarding community involvement anymore.