In my current Drupal project I had the problem, that my client wanted to show the number of all Organic Groups together on the home page. The solution for the problem wrote md on the german Drupalforum. Thanks again. To show the number of any created contents of a content type, just take this small PHP Code and embed it into your drupal homepage or block or anywhere you want:

<?php
$count = db_result(db_query(select count(nid) from {node} where type = "name_of_your_content_type"));
print 'There are' . $count . ' Groups';
?>