I just want to add a little bit to what Oleg said considering conversion-wise vs seo-wise.
One question you should ask yourself when considering what to do is, "What will help my end user decided to actual pay me for my product/service?" In your case, if you're wanting people to actually read the testimonials, shorter, and fewer is better because when someone sees a lot of text they tend to skip over it. This is why top 10 lists are effective as blog posts. The old adage of "you can't judge a book by its cover" should read "you shouldn't judge a book by its cover." I can and do judge by a cover. Magazine publishers have known this for a long time.