If you check your Google rankings and you don't like where you're sitting, it's natural to decide that you need more content. First and foremost, you want to provide something fresh and valuable that readers will click on. Beyond that, you want to get more keywords onto the site in an organic way that shows Google how valuable your site is. That's what helps you rank higher.
Do not assume, though, that you just need to buy some content, post it, and figure you have taken care of the problem. That can provide a quick boost, but it's not going to last. SEO is certainly not a one-time job. You need to focus on consistency and increasing the volume of your content over time.