Whatever it is, it's almost certainly considered a trade secret.

Guess?

Less than 10k units. Probably less than 5k units.

And could be still the best seller at that number in its market.

Hardware synths sell tiny numbers.

The DX7 was an extreme anomaly.

ASM has a runaway hit, in its market, and it's nowhere near these numbers, but has absolutely killed the market for its number one contender, the Novation Peak/Summit.

Any bonafide data?

There's plenty of opinion on the web already. Its why I asked here 😉

I think the chatter available elsewhere is fairly representative of the quality of data you're going to receive here.

This is what I feared.

I thought Yamaha would have wanted to advertise such popularity at every given opportunity.

I know Fender love to herald particular Model's success stories.

This is as close as you'll get ..

Source: Bad Mister https://www.yamahasynth.com/ask-a-question/modifying-a-modx8-ghs-keybed-to-bhe

Extra Credit: ... if you only knew how much R&D goes into these (I’m sure the folks responsible will get a kick out of your post). No single action will please every player, to be sure... but the GH action found in the MODX8 is among the best selling 88 key action on this planet — it’s in the top 2! (I say that on the outside chance there is one I don’t know about). 🙂

There's probably similar comments as breadcrumbs laying around. Not numbers - but notions.

that's a rationalisation for the somewhat annoying "qualities" of the GHS (S = standard, the lowest grade of three GH3, GH and GHS) 88, wherein it's sold in budget piano learning and practicing keyboards, that sell (likely) orders of magnitude more units than Yamaha synths sell. GHS success is not indicative of, nor justified by, MODX sales.

It's a slightly "lifeless" and dull feeling key action, to my ape-like claws.

As to sales volumes... to this day, the MODX exists in a unique field of one. It is unrivalled at its price/feature point, whilst benefitting from Montage perception of value (descended from it being thought of as a Yamaha marquee product), despite its lacklustre sales.

ADDITIONALLY:

The MODX is a good product, way more than 80% of the way to being a great product. User consideration in the software design and implementation is about 95% of that remaining 20% and, sadly, likely never going to happen. The last 20% of software design and development famously takes 80% of the time. Or more.

The other 5% is a screen brightness control. Which ain't gonna happen, either.