Yes, you can optimize for up to 20 main keywords. Make sure to separate each keyword or keyword phrase with a comma [,] or by pressing the shift + enter key together.
For best results, consider planning your content calendar with Keyword Research. Keyword Research helps with identifying topic clusters (groups of related keywords), which you can build Content Editors around.
Keywords in a topic cluster share relevance to each other and help you make sure your content matches search intent you are targeting.