SVRMGR> alter database backup controlfile to trace;
Shutdown the database (immediate or normal - not abort)
*** Take a Full Cold Backup ***
Goto your user dump dest and find the control file produced by the above command (will be a .trc file - sort by date/time to find).
You will need to move/rename your existing control files at this point - also make sure you have sufficient space to create new ones as they will be bigger than the old.
Modify this trace file (call it control.sql or something as well) -
Delete any header information prior to the words Startup nomount.
Increase your maxdatafiles parameter and save the file.
**Shut down the database and take a full offline backup**