I konw there are some plugin about that but you can do that with a simple code. No need plugins. If you want to add some attractive code to show  word count on your posts only edit two file and use a short-code…

Here is the code;

We will change your theme’s functions.php and single.php, because of this wou need to make the backup this files. First the code of  functions.php;

function count_words($str){
     $words = 0;
     $str = eregi_replace(" +", " ", $str);
     $array = explode(" ", $str);
     for($i=0;$i < count($array);$i++)
 	 {
         if (eregi("[0-9A-Za-zÀ-ÖØ-öø-ÿ]", $array[$i]))
             $words++;
     }
     return $words;
 }

Here is the code of sinlge.php;

Word count: <?php echo count_words($post->post_content); ?>

After you placed this code to single.php the code count your words.
That’s all. Have a nice day!