At the moment it's not possible to highlight a section to find out how many words are contained there.

A quick workaround you can try is setting a section as a separate chapter in the program, this will allow you to see the word count in the chapter list.

This is how you can do it:
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1. If you are typing in a certain section of your document set your cursor at the top of the typed area (here I have it in blue).
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2. Set up a chapter above this section.
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You can name it anything you want to. For this example I'll call it "Word Count".

Highlight the title you've added, and click the + icon at the top.

Then click "Save".

3. Find the word count for this section in the chapter list drop down at the top of your Writer's Desk.
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Please note that this count includes the chapter title.