Do not stuff your page. Twenty is probably too many for a medium sized page. Make sure your important words are used often within your text and headers. That shows counts of plain-text words in the first 3000 bytes of your file. That is where most engines put their emphasis when evaluating your page.
Some engines also add extra emphasis to words in the meta tags.
The top 3000 words include the first words your visitor will see. Make sure your most important words appear in the yellow column with more than just a single use. That is what the red column on the right shows. This includes your keyword meta tag and description meta tag . If your most important words appear more too often because they are in the meta keyword tag several times, search engines could consider this keyword spamming and downgrade your rank under those words.