Function for Wordpress that returns the number of active widgets in a sidebar.

get-widgets-count.php

<?php
/**
 * Counts the number of widgets in a widget area.
 *
 * @param $sidebar_index   string The id of the widget area
 * @return int
 */
function get_widgets_count( $sidebar_index ) {

  global $wp_registered_sidebars;

  $index = "{$sidebar_index}";

  $sidebars_widgets = wp_get_sidebars_widgets();
  if ( empty($wp_registered_sidebars[$index]) || !array_key_exists($index, $sidebars_widgets) || !is_array($sidebars_widgets[$index]) || empty($sidebars_widgets[$index]) )
    return 0;

  return count( (array) $sidebars_widgets[$index] );

}