It depends a lot on the content. I've been 3 days doing research for an article and I still haven't even started writing it because I haven't found all the information I need. On the other hand, sometimes I just feel like I have a lot on my mind and I need to put it in writing and I write much more. When I'm hired to write, it's usually 5 articles a day.
Then again, articles have different lengths too. My last article was 6 pages long, for example, so even if it's just 1 article, it might be the equal of 10 400-word articles.