When it comes to adding serrano peppers to chili, the general rule of thumb is to use one pepper per quart of chili. So, if you are making a batch of chili that is four quarts in size, you would use four serrano peppers. Of course, you can always adjust the amount of peppers up or down depending on your personal preferences and the level of heat that you want your chili to have. If you really like things spicy, then feel free to add an extra pepper or two. Conversely, if you prefer a milder chili, then you may want to cut back on the number of serrano peppers used.