Based on my observations, I'd guess that there's quite a large number of D+'s. Since the D+ level was introduced, I've seen on average a steady increase in the numbers using the F checkin and the F lounge, which would most likely be due to D+ members, rather than increased F passengers.

D+ can be earned with just roughly four round trips per year between Korea and the U.S. I think there are a lot of Koreans who make that number of trips per year to the U.S. I personally know of several.

I don't know how they decide on who get's an op-up if there's several D+'s to choose from...whether they can see the total mileage and use that, or based on booking class, or what. But if you're not a D+, my guess is that your chances of an op-up will be quite slim. When I've been seated in economy and get my usual seat blocked next to me, there will typically be another D+ member on the opposite side of the blocked seat. You can spot the D+ or higher passengers, being the cabin manager will usually go around and personally greet them. So in my estimation there will typically be at least a couple of D+ members on each flight.