While you can use the advanced data validation properties of a Google Form question to limit the number of characters in an answer, you will need a regular expression in order to limit the number of words. If you ever have such a need, here it is:
^\s*(\S+\s+){1,9}\S*$
This regular expression will limit the response to 10 words. You can change the 1 and the 9 if you want to have different limits on the number of words (the 1 represents the lowest number your question will accept and the 9 represents the highest).
This is what it looks like in a question:
