How to add order number to multivalue fields classes?
template_preprocess_field(&amp;$variables, $hook) gives you the ability to manipulate fields before they are printed. Just like with template_preprocess_page() and template_preprocess_node(). Not sure what the $hook parameter is for and I have never needed it myself.

Since this is a preprocess function that will be executed for each field, you should add a condition for your field name. Something like this will do:

function THEME_OR_MODULE_preprocess_field(&amp;$variables) {
  if($variables['element']['#field_name'] == 'FIELD_NAME') {
    // Manipulate the data. use dsm() or print_r() to see the array structure.
  }
}


Edit: It would seem as if $hook is always passed as the string 'field', which would be quite useless. Can anyone confirm this?

Edit 2: Okey, so you actually don't need template_preprocess_field() at all if you can override theme_field.

Below is a working example of a extended theme_field($variables) function based of the original version. There is also a good tutorial/example here, which @camcam shared in a comment.:

function THEME_NAME_field($vars) {
  $output = '';
  $i = 0;

  // Render the label, if it's not hidden.
  if (!$vars['label_hidden']) {
    $output .= '&lt;div class="field-label"' . $vars['title_attributes'] . '&gt;' . $vars['label'] . ':&amp;nbsp;&lt;/div&gt;';
  }

  // Render the items.
  $output .= '&lt;div class="field-items"' . $vars['content_attributes'] . '&gt;';
  foreach ($vars['items'] as $delta =&gt; $item) {
    $classes = 'field-item ' . ($delta % 2 ? 'odd' : 'even');

    // Add Row index to classes and iteriate row index variable.
    if ($vars['element']['#field_name'] == 'FIELD_NAME') {
      $classes .= ' row-' . $i;
      $i++;
    }

    $output .= '&lt;div class="' . $classes . '"' . $vars['item_attributes'][$delta] . '&gt;' . drupal_render($item) . '&lt;/div&gt;';    
  }
  $output .= '&lt;/div&gt;';

  // Render the top-level DIV.
  $output = '&lt;div class="' . $vars['classes'] . '"' . $vars['attributes'] . '&gt;' . $output . '&lt;/div&gt;';

  return $output;
}