Rules? WHat rules. Do whatever you find pleasing.

I like to consider the size of my elements of the blocks when I am choosing a border width.

For example, if I have a 3" size 9-patch block, then a 3" or 6" border may look more pleasing than a 4 or 5" border.

If my blocks have elements that are all 4", then a 2", 4" or 6" border might look better than one that is 3".

One inch stop borders look good with many sizes.

If I want a narrow stop border I usually use a flange border. It adds some texture to the quilt a it is only sewn down on one side and is just wide enough to transition from the quilt to the other border.

I go by the size I want and the fabric on which the border will touch, then how much more I want to add. I start off with a 1 1/2 inch or 2 inch, then go on to 6 inches , 4, or 5, again depending on how much more I want/need to add. I measure quilt top 1st , then depending on how much more I need, I will decide on the width and how many of the borders. Never have been one to follow rules, mostly I do what I find is appealing to the eye.

I don't pay attention to any rules or whatever. I make quilts and borders to suit myself. No one person is an expert.
Make as many as you want and make them the size you want. If I add more than one border I try to make each 1-1/2" bigger than the one before. Just me.