H2 headings are used to break up a long post and make it more readable, and there are no rules about how many you should use. In fact, if your post is only a few paragraphs long, then you may not need to add H2 headings at all.
The All in One SEO plugin has a Page Analysis feature that can check your blog post for readability. In general, if you have 300 words of text or more without a heading, it will recommend adding more headings or subheadings.