How to translate plural strings with Drush?
Between the Drush functions that process text, there isn't such a function, but you can implement one using the code from format_plural(), replacing any call to t() with calls to dt().

function drush_plural($count, $singular, $plural, array $args = array(), array $options = array()) {
  $args['@count'] = $count;
  if ($count == 1) {
    return dt($singular, $args, $options);
  }

  // Get the plural index through the gettext formula.
  $index = (function_exists('locale_get_plural')) ? locale_get_plural($count, isset($options['langcode']) ? $options['langcode'] : NULL) : -1;
  // If the index cannot be computed, use the plural as a fallback (which
  // allows for most flexiblity with the replaceable @count value).
  if ($index &lt; 0) {
    return dt($plural, $args, $options);
  }
  else {
    switch ($index) {
      case "0":
        return dt($singular, $args, $options);
      case "1":
        return dt($plural, $args, $options);
      default:
        unset($args['@count']);
        $args['@count[' . $index . ']'] = $count;
        return dt(strtr($plural, array('@count' =&gt; '@count[' . $index . ']')), $args, $options);
    }
  }
}