Qualtrics allows you to validate the content of text entry questions using character sequences called regular expressions (RegExr).
To validate that a text entry question has only one single word:
Select the Text Entry Question in Qualtrics
Click on 'custom validation'
In the pop up, select your question in the first box, and again in the second box
From the drop-down in the third box, select "Matches Regex"
Input the following regex expression which will allow one word (made up of any combination of letters from A to Z and from a - z) without spaces: ^[A-Za-z]+$
Select or create a new error message to display when the condition is not met
If you want to validate on a certain number of words, you can use Richard King's original method to count the number of spaces between words. https://itg.wfu.edu/blog/qualtrics-using-word-count-as-validation-with-text-entry-questions/