Modern formal shirt-stud sets come with four studs. However, I find that I could do with five studs, as the bow tie hides the top button and there always seems to be a sixth button visible near the belt. When the jacket is closed, it's not a problem, but I often open my jacket when seated. Perhaps people can't see it, but I know it's there. Is it incorrect to add an extra stud?
If you want to wear five studs, you are going to have two problems.
First, as you say, they come in fours. And second, the bottom button on your formal shirt is going to be a plain, plastic sewn-on button with no slot for a stud, so you are going to have to have the shirt altered, too.
But I don't see why you would: Whenever you are standing, you will have your jacket closed, so it won't be visible. And when you are seated, the folds of the shirt will cover it. I understand your love of shirt studs, which are so much more luxurious than plastic buttons, but there's no point in going overboard for the sake of a secret.
Russell Smith's latest novel, Girl Crazy, was recently published. Have a fashion question? E-mail firstname.lastname@example.org