This is definitely not a question for the SQL Developer tool forum. The ORA-12801 is considered to be a fairly generic error code. Standard advice is look at other error codes listed in the call stack that follows it, which you do not provide -- not even the number of parallel processes in use, and also in your database alert log. A more appropriate forum could be:
If you have Oracle Support, try MOS Doc ID 1187823.1 (How to Analyze an ORA-12801).
SQL Developer Team