For software sales, I've never seen accurate software sales for any PCE games. It definitely sounds like Tengai Makyou II: Manji Maru is the best-selling CD title, no question, but I'd think the best selling game overall would be a HuCard title since so many more HuCard consoles sold... but maybe not? I don't know.
The rest of this post is about hardware sales numbers, because we actually have those.
This thread is the one to check out:
http://www.neogaf.com/forum/showpost.php?p=149571125&postcount=1
I seem to remember Bill Kunkel speculating in one of his Game Doctor columns late in the Turbo's life that sales of the "various Turbo units" were still "under a million." Seems like a safe guess to me. The Turbo's performance in the States was pathetic.
Between the three major variations of the Turbo in the US the rough number of how many manufactured were. The TG16 (850,000), The TurboExpress (90,000) and the TurboDuo (50,000) (I have no idea how many LaserActive N-10s were made but it wasn't many). So just under a million manufactured , and with a lot of New Old Stock floating around long after the systems mainstream shelf life had passed makes this seem like a pretty accurate statement.
Where did you get those numbers from? I don't remember hearing them before.
For the TG16 -
http://www.gamasutra.com/view/feature/225466/stalled_engine_the_turbografx16_.php?page=1 says 735,000 to 750,000, with at least a hundred thousand of those or more sold overseas later that generation because they never managed to sell them all in the US.
For the Turbo CD / Duo - And as for the Turbo CD, Victor Ireland (of Working Designs at the time, TG16/CD developer) estimated several years ago in a NeoGAF post (I could find it if you want) that he thought that the Turbo CD sold 20,000 systems, and the Turbo Duo also about 20,000 Turbo Duo. He said that with reference to how high an attach rate his Turbo CD games were selling to. Now his estimate may be as of mid '93 when he stopped supporting the system, I'm not sure, but sales after that were not good, the Duo bombed hard unfortunately. To that I'd add an unknown but probably small number of systems sold by TZD from 1994 on, but over 50,000 for the two combined is... unlikely, sadly, I think. Awesome system, it just didn't sell at all.
As for the TurboExpress, never seen numbers for that system before.
For Japan, according to the sales numbers we have, the PC Engine sold 3.92 million HuCard systems. The PC Engine CD sold about a million, and the PC Engine Duo line also sold about a million, adding up to 1.92 million CD and Duo systems combined. These numbers are very plausible, but they do also include too-high numbers for outside-of-Japan sales -- checking the chart (see the link at the top, it's in the first post) it claims 1.77 million PC Engine/TG16 HuCard systems sold outside of Japan, and 100,000 combined CD/Duo systems sold. I believe the lower numbers I mentioned earlier over these.