Add this code to your functions.php file. Make sure you upload 'description-count.js' to the following directory: 'YOUR_THEME_DIRECTORY/js/'

gistfile1.php

 // Add our special description count js
add_action( 'wp_enqueue_scripts', 'our_admin_scripts' );
function our_admin_scripts( $hook ) {
    global $post_type;
    if( tribe_is_community_edit_event_page() ) { // So this only works for posts, would need to edit for events custom post type
        $js_dir = trailingslashit( get_template_directory_uri() ) . 'js/';
        wp_register_script('our-admin-js', $js_dir . 'description-count.js', array('jquery'), null);
        wp_enqueue_script('our-admin-js');
    }
}