Add subcategories posts to the counts column at the admin’s categories list

You can change an array of get_terms() arguments with two filters:

1. get_terms_defaults filters the terms query default arguments;

2. get_terms_args filters the terms query passed arguments.

For example, this code will change the default value for the pad_counts parameter for all queries related to categories and called from the administrative part of the site.

add_filter( 'get_terms_defaults', 'custom_get_terms_defaults', 10, 2 );
function custom_get_terms_defaults( $defaults, $taxonomies ) {
  if ( is_admin() && in_array( 'category', $taxonomies ) ) {
    $defaults['pad_counts'] = true;
  }
  return $defaults;
}