A relatively modern dictionary (I don't know which one, because we've cut out the pages and used them as wallpaper in our bathroom, but I know it's less than 20 years old) indicates that "R's" is one correct plrualization of "R", as is "Rs", but whichever dictionary this is it's kid of a no-name brand, so I'm not sure I trust it.

I've always wondered what the best way to pluralize single characters or numerals ("2's" or "2s"). What's correct?