Drupal 7 how many blocks in region?
I think your best bet would be to use hook_preprocess_region() in a module or theme:

function MYTHEME_preprocess_region(&amp;$vars) {
  // Make sure it's the right region
  if ($vars['region'] == 'sidebar_first') {
    // Get the count of blocks
    $count = count(element_children($vars['elements']));

    // Add the class if necessary
    if ($count &gt; 1) {
      $vars['classes_array'][] = 'some-class';
    }
  }
}