It really all depends on what course I'm playing, but I went with 16+ since thats usually what I keep in the bag most of the time. Don't really need all of the discs on most courses but I like having backups plus the "idea" that its a better workout lugging a heavier bag around.
Usually I carry something along the lines of 5 putters, 5 mid range, 3 fairway drivers, 4 drivers, and 2 throwaway/floating disc for water or hazardous holes that eat discs. Multiples of several molds I like in different states of wear and some as backups if the worst happens and I loss something I really rely on.
I've actually started to just grab 4 discs and carry them without the bag when I play a round with my friends for a lot of casual rounds. Usually a ryhno, a buzzz, a leo and one other disc which depends on what course we are playing.