Determining the no. of arch. redolog files to keep

Hello,

	We have a server which is running Oracle 6 on a Sparc10.  Oracle
	is running in ARCHIVELOG mode, and automatic archiving is currently
	being done.  Offline backups of the database are done at night on a
	daily basis.  We also have two online redo log files.  I noticed
	that the number of archived redo log files keeps growing and are
	taking much disk space, and I would like to delete some of them.
	Given the setup just described, what procedure should I use in 
	determining what and how many archived redo log files should be
	kept?  Should the database be shut down when deleting the files?
	The Oracle RDBMS manual I have only describes how to determine the
	number of arch. redo log files to keep when doing online backups
	of the database.

				-- John