Chili peppers can vary in how spicy they taste, with one jalapeño leaving only a mild tingle in your mouth, while another sets your tongue ablaze. If you accidentally added too much spice to your salsa, you can fix it easily. Remember, too, that the longer a salsa rests for, the spicier it may seem, as the capsaicin in the pepper has a chance to leach out and mingle with the other ingredients.
Adding in extra, un-spiced salsa ingredients to make a larger quantity will reduce the heat in your finished product. For thicker, chunkier salsas, adding extra water can be enough to dilute the salsa to a manageable spice level. If you have extra ingredients, though, making extra will keep your salsa texture the same while reducing the amount of spiciness.
Don't add more than 1 or 2 tablespoons of water for every 1/2 cup of salsa, unless you don't mind noticeably changing the texture of the finished product.