I read this short story in 197x. It consists mostly, perhaps entirely, of a conversation, part of which I can summarize/paraphrase:
“Do you ever wonder why we use base twelve?”
“The advantages of using a highly composite number [...].”
“You don't suppose it's because we have twelve fingers?”
“Oh, that's just a coincidence.”
“It's reasonable to suppose so. But in my research I've found that, thousands of years ago, humanity used base ten almost exclusively.”
“What! Why would our ancestors adopt an obviously inferior base? Well, if so, at least they came to their senses.”
“Here's the remarkable thing, though: there's evidence that, back then, most humans had ten fingers.”
When I read it, I took it to mean that a polydactyl mutation had spread to all humans. A more interesting interpretation is that an unrelated six-fingered species, otherwise resembling us enough to be absorbed in human culture and think of themselves as human, had outlived the five-fingered originals. (Compare Poul Anderson's “Turning Point”, and an analogous story about robots by, maybe, Eric Frank Russell? That's a question for another day.)
Author could be Silverberg, or Asimov, or not. (I may be thinking Asimov only because of a posthuman dialogue in “The Last Question”.)