Right! Your numbers make sense. I was drawing from your list of capitu's fics and cutting duplicates, but the publicly available author-alpha list doesn't have ratings, so that 444 figure includes authors who wrote smut-free works. So I've no doubt that your 208 or 191 figures are more accurate. We could split the difference and say 200 unique authors.

I don't think there's a magic number of authors that would automatically be enough to prevent weighting towards one person's preferences; it's more to do with how much of the sample they represent, so a percentage issue rather than a numeric one. So perhaps it's not a question of adding more authors, but of whether the results change if you limit each author to one fic (selecting at random which fic will be representative of their work)?