If you had zero before it seem to have got 15 ratings. But this not so much the point. You should have gotten the same number of ratings by playing, rating and given feedback to others games, without having to write a 'rate for rate'-post. The issue is that you have to beg for getting those ratings, it should be a natural part of take a active part of game jam. And yes think it I would be a great idea to limit to how many rating you can get, or at least have a limit it until most games has gotten at least 20 ratings.