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

<?php 
   global $wpdb;
   $table_name = $wpdb->prefix . "postmeta"; 
   $login_name = sanitize_user( $_POST['parent_id'] ); 
   $prepare = $wpdb->prepare( "SELECT COUNT(*) FROM $table_name WHERE    meta_value="Purva" AND meta_key = 'parent_id' ", $login_name );
   $myrows = $wpdb->get_results( $prepare );
   echo '<pre>' . print_r( $myrows, true ) . '</pre>';
?>

I tried this and get values but still something missing…