There is no set number of articles you need to write for SEO. What you need to do is write quality articles if you are using them for SEO. It also matters what you are doing with these articles.
If you are using something like SEO Nitro, you will need 600 articles a month for an almost guaranteed ranking on Search Engines. If you are using an Article Spinner, to generate these 600 articles, you will need 8 to 10 articles at 65-75% uniqueness and 300-500 words each to benefit. If you are referring to a personal Blog, just update your blog at least once a month, write quality articles on subjects and with content that is not already played out and you will see a benefit.
(By the way, I do not recommend article spinners or any other black hat SEO tricks to generate massive amounts of content in a short period of time... Blackhat SEO can get you blacklisted and your ranking penalized by Google...and not just the ranking for one site-If you have multiple sites on a server...expect all to see the hit by Google...)
Just a warning and suggestion from someone who has worked with companies that have used Black Hat SEO Tactics...has expertise with article spinners and has worked through such resources such as SEO Nitro ;)