Ceratinly one way round it would be to use the "Insert More Tag" feature on each of your posts. This should be located on the Toolbar between the "Unlink" and the "Proofread Writing" button when you're editing a post. So rather than display the full text of the post, it shows a potion of it (first couple lines, first paragraph, however much you want) with a READ MORE link taking you to the full article. This would enable you to fit more blog entries onto your home page.
Here's an example of a blog that utilizes this:
http://artofmanliness.com/
Other than that, I'm not sure how to display 20 full posts. That might depend on which theme you use.