I have no idea if vgcharz tracks C64 games but I just read in the latest issue of Retro Gamer (number 216, pg 43) that the original Last Ninja games sold 23 million copies on the Commodore 64. This is the quote from the founder of System 3, the developer that created The Last Ninja games. "...the sales of Last Ninja 1, 2 and 3 on the C64 were over 23 million copies. That's quite impressive, because Commodore only sold just over 20 million machines in total."

I'm not sure if this includes The Last Ninja Remix, which was sort of a remake of Last Ninja 2 that was also spoke about in the article.

That sounds impossible.

Yes, it's why I think it must include the remake.

https://archive.org/stream/retro_gamer/RetroGamer_018#page/52/mode/2up

"The original Last Ninja sold a huge amount of units - I think it reached 4 million on the C64. Ninja 2 came out right at the point when the C64 was at its most successful in terms of sales, and sold 5.5 million copies. If you take that into context, it’s estimated that around 20 million C64S were sold, so one in four C64 owners bought the game."

Back in 2005, he put the combined sales of the three titles at 12.5 million (he says three million for the third later on). I think it's safe to say that it has not sold any more copies since 2005. The remix of the first existed at that time, so there's no reason he wouldn't have included it then, especially if it sold so incredibly well (over 10 million if the 23 million figure is to be believed).

There are a few potential explanations, but the most likely is simply that the dude is exaggerating. I wouldn't even take the 12.5 million figure too seriously. 

I do think he added all platforms together to get to 23M. While it started on the C64, it was also on a slew of other platforms:

From what I gather, those 12.5M were only for the C64 version, and the 23M could be all versions combined, which sounds plausible to me. The C64 versions were very well loved and some of it's Killer Apps, so reaching very high sales numbers is certainly not out of the question.

While true, that actually only came later in life (it was only for the diskettes, and until 1987 3'5" diskettes were pretty expensive and copying games thus not exactly much cheaper than just buying them) and didn't concern cassettes all that much, hence why there's a steep drop in sales between TLN 2 and 3

"...the sales of Last Ninja 1, 2 and 3 on the C64 were over 23 million copies. That's quite impressive, because Commodore only sold just over 20 million machines in total."

He specifies the Commodore 64, and doubles down by mentioning the commodore 64 install base. Either he made a massive error in speaking, or he's lying. 

And, 10 million would be a lot for those other systems especially because they were all incredibly niche products except for PC and maybe Apple. Collectively, the games would have to have averaged around 1,000,000 on each of those platforms, which would be incredibly high sales.  

23 million is a pretty staggering feat. To put it in perspective, that's a little more what the original crash Bandicoot did on the PS1. So... to believe that The Last Ninja sold that much, I'm gonna need a lot more than its developer's word, especially when he is at best not great at communicating the sales and at worst telling a fish story.

They were no big sellers by today's standards, that's for sure (well, apart from the NES). And it's also quite possible he's exaggerating, I certainly won't deny that. I just gave you a possible reasoning as to how to get to 23M from previously 12.5M, that's all.