Unsubscribed from Simplenews Newsletter is there a way to find how many users unsubscribed from my Simplenews newsletter at my Drupal-7 website? And, if yes, can I detect from which particular newsletter did the user unsubscribe from?
You can click admin/people/simplenews and check on status column.
OR
you can use following code to get the data

  // fetching from the database.
  $query = db_select('simplenews_subscriber', 'ss');
  $query-&gt;innerJoin('simplenews_subscription', 'si', 'si.snid = ss.snid');
  $query-&gt;fields('ss', array('mail'))
    -&gt;condition('ss.activated', $condition_active)
    -&gt;condition('si.status', $condition_subscribed)
    -&gt;condition('si.newsletter_id', $newsletters)
    -&gt;distinct();
  $data = $query-&gt;execute()-&gt;fetchAll();


where si.status is a flag indicating whether the user is subscribed (1) or unsubscribed (0)

and $newsletters is a Array of newsletter ids to filter on