There's already a Users online module you could use. Take a look in skin1/modules/Users_online/
To get the total number of registered customers in your database
Code:
$result = mysql_query("SELECT login FROM $sql_tbl[customers] WHERE usertype='C'");
$total_registered_users = mysql_num_rows($result);
$smarty->assign("total_registered_users", $total_registered_users);
P.S. You should put your X-Cart version in your forum signature