Does user_is_logged_in() check if there is at least one user logged in?
By looking at the Drupal API, you can see the function itself:

&lt;?php
function user_is_logged_in() {
  return (bool) $GLOBALS['user']-&gt;uid;
}
?&gt;


Which shows that it returns TRUE if the current session is a logged-in user, or FALSE if the current session is an anonymous user.