The error implies that you have hit a limit of some form. This is usually an OS limit that has been reached.

Typical problems are:
- Out of system memory / swap
- Out of process slots in the process table
- Streams resources depleted
- Out of File Handles

If the init.ora 'processes' parameter seems ok then you
may need to increase the Unix kernel parameters for the
maximum number of processes or users (for example, nproc
or maxuprc). Check the manuals for the Unix operating
system for more information on these parameters.