How many 1966 C-15 (long wheelbase) Stepside trucks were made?

At least one because you own it...

Sorry for that, I couldn't resist. Check the web and you should be able to find the answer you are seeking.

I took a "quick" perusal and you will need to research in more detail. The sites that I checked did not post manufacture numbers but also had them classed by generation. 1960-1966 is first generation. After that it will all boil down to options. If you dig deep enough you should find all the data you need right down to how many had whatever engine, and how many had two toned paint. Cars that are highly sought after (mainly muscle cars and/or high priced vehicles) usually have specific numbers but these are usually the result of an enthusiast doing detailed research which can add value to their vehicle should they decide to part with it.

Went to an exotic auto auction some years ago and they auctioned off 2 like looking Studebaker Avanti's. Because one of them was oddly optioned it brought about 25% more of an auction price then its sibling. That is on top of the fact that the other (lesser priced) car was lower mileage and a lot cleaner...