Looks like you're right Murray. From your table, even if we add another 14 cars/day to your 171/day estimate, to get to the 185/day GM is estimating, over the remaining 20 days to finish at December 7, that results in another 280 cars and adding them to your 18,865 December 7 number, gives us 19,145 which is 1,036 short of the goal of 20,181. That's short by about 6 production days if I'm doing my math correctly. And that assumes no slowdowns to the 185/day which is historically a high number. Perhaps (?) they intend to run overtime or work on weekends? I'd hate to be someone waiting for a 2020 at 3000 status and one of the last ones produced. If GM stops making 2020's on December 7, they may not get their C8.
But Andy correctly caught the comment in the article, that they may produce both 2020's and 2021's at the same time. If so, maybe they'll time it so any 2020 is built by the end of the year. Seems somewhat odd to juggle the 2 production years that way.