The heat level of these margaritas will depend a lot on how hot your particular jalapeños are, which can vary a lot in their spice level. So, if you like, you could taste a tiny piece of the raw jalapeño before making the drink, and decide how much to add depending on the heat level of that particular pepper. To scale down the heat from what is written in the recipe, use one slice of jalapeño rather than two. You could also remove the ribs and seeds before muddling the pepper. To scale up the heat, use three or even four slices of jalapeño. For major heat lovers out there, you could even try adding a small piece of a pepper that is slightly hotter, such as a serrano. (If you are sensitive to heat however, please be careful because the serrano is 3 - 9 times hotter than a jalapeño! Here is a good article that describes the heat of a serrano vs. jalapeño.)