functions.php

<?php
/**
 * Add row classes to the archive pages
 *
 * @param array $classes
 * @return array $classes
 */
function be_row_classes( $classes ) {
  // Only run on archive pages
  if( is_singular() )
    return $classes;
    
  global $wp_query;
  if( 0 == $wp_query->current_post )
    $classes[] = 'row-1';
  if( 1 == $wp_query->current_post )
    $classes[] = 'row-2';
  if( 2 == $wp_query->current_post )
    $classes[] = 'row-3';
  if( 3 == $wp_query->current_post )
    $classes[] = 'row-4';
  if( in_array( $wp_query->current_post, array( 4, 5 ) )
    $classes[] = 'row-5';
  if( in_array( $wp_query->current_post, array( 6, 7 ) )
    $classes[] = 'row-6';
    
  return $classes;
}
add_filter( 'body_class', 'be_row_classes' );