I would focus mostly on creating new content.
Depending on your niche, I would focus on internal linking using new content and I would also continue publishing until I reach at least 150 posts.
Again, this depends on the niche.
But generally speaking 20 articles is simply not enough.
Sure, your blog may rank fairly quickly even with 20 articles, but it really depends on what your competitors are doing.
It also depends on the quality of the content that is already out there.
For instance, one of my blogs that focuses on a fairly obscure topic with decent monthly search volume skyrocketed in Google rankings fairly quickly.
And I only had about 50 posts up on that blog.
But as more competitors entered the space I noticed that it's ranking started to go down.
This is natural.
So I started publishing more content and interlinking more aggressively in order to lead to a decent recovery.
You have to think along the lines of just how "complete" your site is.
20 articles is simply not enough.
Also don't think that buying backlinks is a viable alternative to producing high value responsive content on topic content.