Depends on where you are, I guess. Here in Silicon Valley it's pretty significant. Though many of the people I know who could retire keep working. It's truly what they enjoy doing, apparently.

One odd thing about this is that you'll rarely meet the rich & retired in your work life, by definition, so it's easy to underestimate how many they are. That said, I have no good way to count them either, and may well be overestimating.