First of all, NEVER hire a writer who you will be paying per working hours, because that never goes well. I started like this and it is really annoying when your writer has a bad day and writes like he is in kindergarten - and you still have to pay him because you are paying per hour, and not per work done.
As of the maximum, there will be people stating that they can write up to 25 articles per day - but they can't, they just found a few more buddies and will write it together. A "healthy" per day number is around 15x500 words articles, all over that is plain shit.
And also, you should find one writer for the "no-brain" job like writing random SEO articles which need bullshiting and filling, and a "I-use-my-brain" job for linkbait content where the writer has to spend some time researching it.
In the beginning I wanted less writers and more articles, today I want more writers and less articles. I would rather see my writer write 10 articles par day which are GREAT, then bullshiting his way out with 20 articles per day.