I voted 16-20. My casual round bag is actually when I max out on discs. I carry everything for casual rounds because sometimes I like to throw extras and try different lines and shots that I don't usually throw.
For tournaments I usually prepare the night before by going over the holes in my head and figuring out what I will throw for each shot, then taking out whatever discs I won't be using. My typical tournament bag consists of 9-15 discs, but sometimes I max out to 20 if it's a good course that requires all sorts of lines and shots.