A good rule of thumb is to focus on one or two long-tail keywords per blog post. While you can use more than one keyword in a single post, keep the focus of the post narrow enough to allow you to spend time optimizing for just one or two keywords.
Use Google Trends to find the most searched keywords on Google. Go to
https://trends.google.com to find the most popular searches around the world. Google Trends also helps you find trending searches for each day, week, month, and year for thousands of keywords and topics.