I can think of 3 reasons a disc is in my bag:
1. Staple. I have probably 8 discs that get the call most of the time.
2. Auditions. There's always a couple that seduce me in the field that are trying for a permanent spot.
3. Situational/utility. While it's awesome to be able to throw a perfect sky anhyzer with your Roc/Comet/Drone/whatever to get out of trouble, sometimes life's just a lot easier if you've got a Champ Whippet and a passable forehand.
As my game's developed into a higher state of mediocrity, the number of staples has decreased, the number of auditions at a time is down (but there's still always something around to have a short, intense love affair with), and the situationals have been reduced pretty far. I imagine as the process continues, these numbers will keep going down. (Then again, maybe as I get older I'll buy a sweet cart and anything with a nice dye or a special swirl in the plastic will get a ride.)
Right now, for a tourney I'm showing up to the course with 25 or so to cover contingencies for rain/wind/etc, carrying around 15. Of the 15, I'm going to 8 for most of my shots, 3 are spares and the rest are utility.