One comment, one good comment in a relevant post with PR per blog will be enough in my opinion. However, if there are more relevant posts in the same blog, and chance to put a comment, put the comment using another keyword/page in your site. It's said that G only sees the first link. What I mean is: if I have 3 links in a blog post: mykeyword--> linked to www. example. com; mykeyword1--> linked to www. example. com; mykeyword3--> linked to www. example. com. The first one will be more valuable. Just my two cents.