I think it depends on the content as much as the quantity.
Let's say I have a site called BlueWidgetReview dot com. If I have 4 or 5 articles which successful review the blue widget, I'm A-okay by Google. I won't get badly penalized. I will only have to write more articles about the blue widget if my competition has more of the same content that answers this query (Google won't penalize me in isolation, if you see what I mean.)
I find that it boils down to ensuring that you ANSWER the question denoted by the website. If you do it successfully then the number of articles is determined by competing sites who are doing the same thing as you.
Saying all that, I try to add lots of content. I can whip up a Panda-loving article in about 10 mins. But I never just add fluff, padding. I always try to answer the question of my site.