Glyphicons either come as an image file or font set. Either way, they are not extensible. They are individual icons. It's like saying, I know there's a character set of a-z, but can it also have a icon of a puppy. Or I know this is an image of an apple, but can the pixels change so it's a number.
You can, of course, easily do this with CSS. It's a trivial implementation, so I wouldn't worry about the 'cost of extra classes'. If you're developing a website, you should have plenty of classes.
In short, no. Here are all the available glyphicons.
Glyphicons either come as an image file or font set. Either way, they are not extensible. They are individual icons. It's like saying, I know there's a character set of a-z, but can it also have a icon of a puppy. Or I know this is an image of an apple, but can the pixels change so it's a number.
You can, of course, easily do this with CSS. It's a trivial implementation, so I wouldn't worry about the 'cost of extra classes'. If you're developing a website, you should have plenty of classes.
Try any of the suggestions from how to use CSS to surround a number with a circle?:
Use Border Radius
Use Unicode Characters
For 20 and lower, you can just use the enclosed_alphanumerics unicode characters:
① ② ③ ④ ⑤ ⑥
Demo in jsFiddle