Cary,
Its a good question, and one any gca should answer generally before starting a design, and then, perhaps change the answer in relation to topography. If you base your philosophy on how it plays for golfers over some other notion (which you should!) then its a fair question.
My answer is a minimum of 14, probably 16, (one per side) and maybe 15, with one each on a par 3, 4, and 5.
Lately, I have been trending closer to 18. After restoring La Costa to something close to the original Wilson design, and seeing too many golfers get in those green side bunkers, the members asked us to screw Wilson and make the second course easier. In playing with them and the media, on nearly every hole I saw someone hit a front bunker (or pinching lateral front bunker) which was no fun. On almost every hole, if we could have moved the side bunkers out and back just a yard, fewer golfers hit them.
For the average golfer, a "good shot" isn't being in the right portion of the green, etc. It's one that got airborne, flew about in the direction of the green with enough distance to get there.....at least almost. For them, having a good shot land in a frontal bunker or pond just doesn't seem fair. And, "them" make up 90+% of golfers.