You're doing a huge disservice to the environment by using CD
-R discs extensively, no matter how cheap they are. Please switch to less wasteful media.
Not to mention that they degrade, and you'll have to keep checking them to discover which have gone bad. You're better off having another hard drive in a separate computer. When one breaks, you just duplicate the other one. For more redundancy, use three, etc.
Of course, if you're *really* paranoid, you'll have offsite backups on another continent, but that's another story...
Finally, to answer the question, I use about 10 CD
-R discs a year, mostly to transfer stuff to people without proper network connections.