I did a habenero beer last winter, and found that about 1 pepper per gallon gave a nice charachter, without being too overbearing. Jalapenos are a larger pepper, but since they are much more mild, the same ratio may apply. I would reccomend roasting the peppers, that will give you more of the flavor, and less of the heat, even though the heat will still come through. Be careful, because the roasting releases more of the oil in the peppers, and too much could impact the head retention. Let me know how it turns out!