Because words can consists of different lengths of characters it is easier to limit the entry to the number of characters.
A form field has a size in pixels. Letter take up a certain number of pixels but not a fixed number..
How many letters are there in a 20 word sentence?
Since there could be any number from 20 to over 50 or more, this approach might not work.
What size filed will be needed for 20 words?
Now exactly how are you going to count word?
Using white spaces? How will you handle hyphenated words?
Using a dictionary lookup? Where will the word list be kept?
Acrobat form fields are not by default dynamic. Will you adjust the size of the field based on an estimated size of the text?