They're rare, but I'm beginning to think they're not as rare as a lot of people seem to think.
There are some widespread misconceptions out there. The story everyone knows is that there were about 2000 red radar scopes that were converted to DKs. These are the TKG2-UP models. BUT . . . after that, Nintendo made quite a few Red DKs that started life as DKs at the factory. These were not radar scope conversions. These are models are labeled TKG3-UP. There are also some blue TKG3-UP models.
The serial numbers are not that great of a guide becuase its not clear what system they were using and there look to be two seperate serial number systems for the early models. From what I can tell, there were somewhere between 3,000 and 5,000 red DKs (this is by no means set in stone). Assuming people are right that 60,000 DKs were produced that's somewhere between 5% and 8% red DKs.
Look at the linked thread for more info. I do think it's pretty weird that the last TKG3 serial number we have is 7,791 and the next one we have after that is 30,467. The latest DK we have on our list is 165,105 (ie. 65,105 because Nintendo started putting a "1" in front of the serial numbers). It almost looks like they stopped the TKG3-UP around 8,000 and started the TKG4-UP at 30,000. Meaning that the assumed production numbers could be about 20,000 off.
I have to go now, some football playeers are giving me a swirley at 4:30.