Custom function that takes a WordPress post ID and returns the number of words in the post.

count-post-words.php

<?php
/**
 * Count Post Words. 
 *
 * Custom function that takes a WordPress post ID and returns the number of words in the post.
 * You should paste it on your theme's functions.php file. 
 *
 * @link https://imelgrat.me/wordpress/customize-wordpress-post-management-page/
 *
 * @param int   $post_id The Post ID
 * 
 * @return int  The word count.
 */

function post_word_count($post_id) {
    $content = get_post_field( 'post_content', $post_id );
    $word_count = str_word_count( strip_tags( strip_shortcodes($content) ) );
    return $word_count;
}

?>