You can only choose one keyword. You can add synonyms to optimize for as well. You can add this in the editor, next to the keyword.
Synonym literally means “word with (almost) the same meaning”. Synonyms can also be related to search results. Suppose you search on the word “pictures”, a related search result can be “photos”. When choosing keywords for your website, it is very important to consider the corresponding synonyms of the keyword and how well they will fit into your web content.
Select the highlighted spots in the image below to set up your synonyms.
Watch video on how to use Text Editor (including Optimize tool):