ACF Relationship: Count of Related Posts

Looking to get the number of CPT B posts related to a CPT A post when using a Relationship field with ACF?

Here’s a custom function for that:

/**
 * Get the count of related posts for the current post
 * or a specific post ID
 *
 * @param string $field_name The relationship field name slug.
 * @param int $post_id Optional. The ID of the post. Defaults to current post ID.
 * @return int Number of related posts
 */
function bl_get_related_posts_count( string $field_name, int $post_id = 0 ): int {
  $post_id = $post_id ?: get_the_ID();

  $related_posts = get_field( $field_name, $post_id );
  
  if ( ! $related_posts || ! is_array( $related_posts ) ) {
	return 0;
  }
  
  return count( $related_posts );
}

bl_get_related_posts_count( 'related_cities' ), for example, can be used to output the number of cities for each country in a country query loop where related_cities is the Relationship field name.

To use this in Bricks with echo dynamic data tag, whitelist it:

add_filter( 'bricks/code/echo_function_names', function() {
  return [
    'bl_get_related_posts_count'
  ];
} );

and then

{echo:bl_get_related_posts_count(related_cities)}