This error messages indicates that the max number of processes set by the processes parameter is reached. You can check the number of processes in a Oracle database by executing the following SQL query:
select count(1) from v$process
This will return you the current number of processes, to check the maximum number of processes you will have to check the parameter set by executing the following command:
SHOW PARAMETERS WHERE NAME = 'processes'
If you feel that this number is to low you will have to change it which can be done by executing the following command:
alter system set PROCESSES=600 scope=SPFILE;
600 is the new value of the processes parameter. For more information about the alter system options and alike check this website.