As many as I can get away with! In all seriousness, it really depends on what's out there to write. I usually find assignments to do rather than writing something and trying to sell it. So, I check around at the different places I work at to see if there's anything interesting, easy or high paying for the work involved.
Since I can't give you a solid number of articles, I would wager that I write somewhere between 1,500 and 2,000 words per day on average. The amount per word is impossible to determine since the pay rates are quite varied depending on which site I'm writing for.