How to count data records in wordpress which have same meta_value in wp_postmeta table?

I want to get number or count of same meta_value of meta_key == ‘parent_id’.How can i get it?
What should be the wordpress query for counting records in same table?

//set the meta_key to the appropriate custom field meta key
$meta_key = 'parent_id';

$allmiles = $wpdb->get_var( $wpdb->prepare( 
  "SELECT meta_value,
  SUM(IF(meta_value = "Purva", 1,0)) AS 'parent_id',
  COUNT(meta_value) AS 'total'
  FROM $wpdb->wp_postmeta
  GROUP BY post_id
  ORDER BY parent_id DESC", 
  $meta_key
) );

echo "<p>Total miles is {$allmiles}</p>";