How to show current utilization of sessions, processes and other important settings for a database

Example:

set lines 300
col RESOURCE_NAME format a30
select RESOURCE_NAME,CURRENT_UTILIZATION,MAX_UTILIZATION,INITIAL_ALLOCATION,LIMIT_VALUE
from v$resource_limit
where resource_name in ('processes','sessions');

RESOURCE_NAME                  CURRENT_UTILIZATION MAX_UTILIZATION INITIAL_ALLOCATION     LIMIT_VALUE
------------------------------ ------------------- --------------- ---------------------- ---------------
processes                                     1498            1500       1500             1500
sessions                                      1511            1517       2272             2272

Documented for Oracle 19c here

How to pause an Oracle database

Sometimes it is useful to pause a database, and let it finish the running jobs at a later time.
Particularly in environments with shared resources contention may occur. Important databases need to be prioritized while less important ones should be suspended.

For this purpose, you can use the "suspend" command:

alter system suspend;

Alert log:

Mon Oct 05 10:14:49 2015
Suspending database after alter system suspend command

The database_status field in v$instance will reflect the new state of the database:

SQL> select database_status from v$instance;

 DATABASE_STATUS
-----------------

SUSPENDED

When you are ready to resume database activity, issue the following command:

alter system resume;

More info: Oracle Documentation