Checking the maximum number of simultaneous connections is possible in the dictionary:

SELECT NAME, VALUE FROM V $ PARAMETER WHERE NAME = ‘sessions’;

Checking how many connections are currently connected is possible in this way:

SELECT COUNT (*) FROM V $ SESSION;

Obviously, both things are checked at the system level.

Of course, you can change the maximum number of connections:

ALTER SYSTEM SET SESSIONS = 400 SCOPE = SPFILE;

Configuration changes will take effect after the reboot.