ORA-12516 and ORA-12520

ORA-12516 -> check lsnrctl status and services to make sure the instances are registered..

lsnrctl status;
lsnrctl services;

Make sure the service you want to connect (for exampla PROD1) is not a blocked state (state:blocked)

ORA-12520 -> Process limit reached.
Check the process values with the following sql;

select * from v$resource_limit where resource_name in ('processes','sessions');

For RAC.

Check lsnrctl services , and lsnrctl status and also check process limit..
If found any problems in service registration, check local listener of the rac nodes.(sqlplus > show parameter local_listener)
Local listener should be set to vip addresses..
For instance 1 Local_listener Should be like;

local_listener       string      (ADDRESS=(PROTOCOL=tcp)(HOST=erm1-vip)(PORT=1521))