50 posts within 7 days might lead to spamming across the forum; you must need to reconsider it.
Why?
Don't we have campaign which allow you to post up to 55 a week for individuals? I do not see anything wrong here if users agree to make whatever post they have asked for.
Even 100 per week in pay per post model will not enforce participants to spam. But, higher post requirements in fixed/weekly model may lead you to chase your post count even you do not have unique and valid points to contribute in a discussion.