I'm a "the more, the merrier" girl. Love Bonnie Hunter's quilts, where she will use 4 or so colors, plus a neutral, but within those colors, she uses the full range. So reds might be almost pink through burnt oranges to fire engine red.
Are you using a pattern? That would give you how many actual colors you need, although you might use more than one fabric to get to required yardage. It's really in your taste; do you like "scrappy" or does that just look like chaos to you?