If you increase number of articles on your site but decrease quality of them then what would be the effects in SEO?
Well your readership would decline if your content quality declines. So your SEO will decline too as far as getting good ranking positions through shares, social signals, mentions, citations, backlinks etc go.
Great content naturally gets linked to a lot, gets a lot of shares etc by the readers.
Poor quality content doesn't.
So even if you increase the articles but are decreasing the quality. All you are doing is peddling the same rubbish articles nobody wants to read.
Where as if you increase the articles you publish to your site and also increase their quality then you are posting quality articles that people want to read and share with their next door neighbour.
Even if you are writing articles for another site that links to your site where you have better quality articles, you still don't want to make those articles crappy low quality articles as otherwise those articles wont get much chance to rank high and nobody will want to read them and visit your site through them anyway.
Therefore, it makes more sense to always post high quality articles, whether its to your own site or on another site.
That way people know your site is a quality site before they've even visited it.
But then when they do, they realize its actually better than they expected because all your content is just super amazing high quality.
It's a no brainer if you ask me.