How to query the count of oracle connection ?

 SELECT s.username AS username,
  (
  CASE
    WHEN grouping(s.machine) = 1
    THEN '**** All Machines ****'
    ELSE s.machine
  END)     AS machine,
  COUNT(*) AS session_count
FROM v$session s,
  v$process p
WHERE s.paddr   = p.addr
AND s.username IS NOT NULL
GROUP BY rollup (s.username, s.machine)
ORDER BY s.username,
  s.machine;