I still think Asahi Newspaper's 5.8 million global sales quote is probably accurate, or at least very close.
I checked again, and it appears that Famitsu's domestic Japanese numbers, 3.92 million HuCard systems and 1.92 million CD systems, puts Duos exclusively in the CD systems group. Searching for info about Duo sales in particular, I found
this webpage which says that the Duo systems sold about one million (and that the CD-ROM ad-ons sold about one million, which fits the 1.92 figure well). That seems reasonable to me.
So that would mean that Japan has just under five million independently playable systems, just under two million of which can play CD-ROM games. Put that together with Asahi's figure and the US has somewhere around 1 million. That sounds reasonable to me.
it's probable that the pc engine units sold in japan was between 6-8 million on the basis that megadrive had installed user base of 3.5million ( distant third) in japan and based on accounts, pc engine was very popular and second to super famicom which had 17 million user installed base in japan but by references over the years, megadrive was a distant third in their home market, nowadays, even buying both consoles and games for megadrive from Asian countries and japan are not that common unlike pc engine and super famicom from online auctions and physical stores.
It's a fallacy to assume that the sales of the 2nd place contender lies right in the middle of the 1st and the 3rd. Nintendo walloped everybody back then in sales, both with the Famicom and the Super Famicom. This is obvious when looking in used game stores and Goodwill-style recycle stores. Nintendo stuff is everywhere.
Also, I think that in 89, 90 and 91, the PC Engine was much stronger, but the Mega Drive crept up on it in the later years. 3.5 million Megadrives, 3.9 million PC Engines and another million Duos seems very believable to me based on availability in Japan.
You also can't really say much by comparing import demand for the systems, except that the Genesis was more popular in the US than Japan, and the PCE was more popular in Japan than the US. Not to mention, many Mega Drive games are region locked in a way that can't be defeated with a simple cart-port mod or a converter.
You also got to consider Taiwan, Hong Kong and unofficially china as well as licensed ver of pc engine in korea contributing a million here and there, as well as few tens of thousands unofficially imported to European countries
I would be shocked if sales in all regions other than the US/Canada and Japan exceeded even half a million. I wouldn't be surprised if it's not even 100k. The Korean system is incredibly hard to come by. I mean, systems that sold in the 50-100k region (PC-FX, Supergrafx, Playdia) are far, far easier to locate.