I thought it was the former, but in my last game I only had 18 ratings in the "Humor" category and still got a score. Kinda worried now, because I don't want to miss out on a score and I still only have 19 ratings in humor this time around...

Edit: To clarify - I meant to ask whether you need 20 ratings to get a score in the corresponding category (i.e. are 19 humor ratings enough if the average is >20?)