word count is not always the right option to improve your rankings (actually, it's rarely the right option). Check what your competition is doing and try to copy them. If everyone who ranks in top 10 has 2000 words on average, but you have 800 words then probably that you need more content. But that's rare.
But if everyone (or most of them anyway) has 600 words and you have 500 words the content length might not be the problem, but it might be an optimization issue (you don't have enough internal links pointing to the article you wish to rank in top 10, or the internal links might not have the right anchor text, or you might not have OBLs, or not enough h2 / h3 / h4 tags, or your image might miss the keyword in its ALT tag and filename, etc) There might be many reasons why you're not ranking in top 10, and content length is just one of them. You might need a few good backlinks, or your domain name might not have enough topical relevance and / or authority to outrank your competition (this is especially true in tough niches like finance, medical, web hosting, SEO, health, etc)
Basically, you need to have a good look at your competition and check how many words their content has, how well optimized it is, how well their content addresses the intent behind the keyword, how many RDs their domains have, how big their domain's DR is, how many backlinks they have, how topically relevant their domain is to the niche, etc....
