Word count for a single topic:

Select the topic, then select the Word Count tool in Write > Font.

Word count for the entire project:

Select the Report tool in the Project tab and choose the Full Report option. This will include a word and character count at the end of the report, and individual word counts for each topic.

You can include/exclude topics in/from the entire report and the word count by selecting or deselecting Include Options and the other filtering options in the report dialog (topic classes, topic status, modification date range).