Welcome to the board from Southern California!
There are no rules. I usually prefer two borders. I will use something contrasting as a narrow border anywhere from 1 1/2 to 2" and then a wider border of the main focus fabric if I have it. I personally don't like really wide borders. I will usually go with a 4 or 4 1/2" to 6" outer border. It depends on how much quilting you want to do in the border. A lot of this is personal preference or what you have enough fabric to do. I can't tell you how many times I have measured my remaining fabrics and once I figured how many strips I need I divide the whole piece buy the number of strips and that is how wide the borders are.