The law of diminishing returns applies. Most people find that pending on their camera, conditions, and etc the balance is around 40-50 images. I have done as much as 170 when testing this. Here's the kicker, MORE SHOTS can REDUCE the quality of your final product if you introduce not so good shots in the stack. The best bet is to shoot MORE then you need, and pick the best subset you can. In that stack of 170, I actaully got better results when I threw out 50 or so images.
WebCam users shoot in to the 1000's of shots to stack, but that is a different game, and the rule still applies, just at a different scale.