To monitor the connections used in Oracle, you can use below SQL to get the data.

Get current connections and percentage of used connection to max connections:

SELECT I.INSTANCE_NAME,
       I.HOST_NAME,
       COUNT(*) CONNECTIONS,
       ROUND((COUNT(*) / P.VALUE * 100),2) AS PERCENT_USED
FROM GV$SESSION S, GV$INSTANCE I, GV$PARAMETER P
WHERE S.INST_ID = I.INST_ID AND I.INST_ID = P.INST_ID
  AND USERNAME = 'YOUR_USER'
  AND P.NAME = 'sessions'
GROUP BY I.HOST_NAME, I.INSTANCE_NAME, P.VALUE
ORDER BY I.HOST_NAME;

Get connections count for each machine:

SELECT MACHINE, COUNT(*)
FROM GV$SESSION S, GV$INSTANCE I, GV$PARAMETER P
WHERE S.INST_ID = I.INST_ID AND I.INST_ID = P.INST_ID
  AND USERNAME = 'YOUR_USER'
GROUP BY MACHINE;