Limiting number of related posts

Build an array of categories:

foreach ($categories as $category) {
    $cat[] = $category->cat_ID;
}

Then pass that to the query:

$args=array(
  'category__in' => $cat, 
  'order' =>DESC, 
  'post__not_in' => array($post->ID),
  'posts_per_page'=>2, 
);

In other words, put the query after the foreach, not inside of it.