I'm wondering what the optimal number of people on the leaderboard would be.

From a usability point of view, 15 sounds about right to me. There is already a lot of other stuff on the side bar.

I suspect that if there were 20 people on the leaderboard, that would increase the motivation effect, without significantly devaluing being on the leaderboard itself.

Maybe, if the point is to increase motivation there might be better ways. I don't know if these are any good, but here's some example ideas:

• Designated appreciators - people who try to highlight the good parts in posts and offer constructive advice on how to improve them.
• Month in review post - this would be a post that contains the authors favorite posts last month with short abstracts of the posts.
• A more flexible feedback system.
• Some designated (nice) users who offer to give advice if people message them.
• Some designated collaborators - if you have some ideas for a post, but don't know enough about the subject you can message some users who have volunteered and have knowledge in that particular area. If they have time, they can help you find what areas to look into further and offer advice on how to improve your post.
• Suggested articles post - a post which provides some ideas that look interesting and haven't really been explored thoroughly yet. This would be good for people who don't have any ideas on what to write for a post.
• Perhaps, integrate the karma awards for the users who volunteer to take on certain roles.