To count concurrent users in Oracle Apps

select count(distinct d.user_name)
from apps.fnd_logins a, v$session b, v$process c, apps.fnd_user d
where b.paddr = c.addr
and a.pid=c.pid
and a.spid = b.process
and d.user_id = a.user_id
and (d.user_name = 'USER_NAME' OR 1=1);