ORA-12516: TNS: listener could not find available handler with matching protocol stack

On one of our database appears error when users are trying to connect to database:

ORA-12516: TNS: listener could not find available handler with matching protocol stack

What does it mean? It's mean that our listener can't find available hanlde. Let's check into database how many processes we have:

sqlplus / as sysdba

SQL*Plus: Release 19.0.0.0.0 - Production on Thu Jun 4 11:51:25 2020
Version 19.3.0.0.0

Copyright (c) 1982, 2019, Oracle.  All rights reserved.


Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.3.0.0.0


SQL> select limit_value, current_utilization, max_utilization from v$resource_limit 
     where resource_name='processes';

LIMIT_VALUE                              CURRENT_UTILIZATION MAX_UTILIZATION
---------------------------------------- ------------------- ---------------
       300                                               295             300

SQL> 

This is reason because oracle can't make server process for corresponding user process. Solution Just Increase parameter processes and restart database.

alter system set processes=500 scope=spfile;
shutdown immediate;
startup;