Sure, will put it on my TODO list. Since I know which cards are available as Shiny Pokémon now, looking for how many cards for each of those shouldn’t be too hard. And most I already know when I was searching for the best-looking but still affordable copy.
Greetz,
Quuador
@queendrifloon the list is done. I’ve only included one of every artwork, either in English or Japanese exclusive. Most cards have multiple variations like Reverse Holos and stamped versions and such, but I’ve only included one of every artwork. I’ve also included three other cards featuring a Shiny Pokémon that I’m aware off, which are within double parenthesis. Here it is.And you’re indeed right that Altaria has 4, Mew has 3, and Rayquaza has 7. 
Gyarados and Rayquaza have the most, both tied with 7 unique Shiny artworks.
Greetz,
Quuador
This is right up my alley; thank you as always!
Ah, you missed one for Poipole;
I only noticed because I love shiny Poipole’s color scheme so I have this card myself (not this PSA one, but I’m not home to photograph my copy)
Ah, yes, I indeed didn’t knew about that one. Tbh, I wouldn’t be surprised if more Japanese exclusives were missing. I used this list for the English cards. And I know about the Shiny Altaria XY-P promo and GX Ultra Shiny set, but no doubt there are more Japanese exclusive Shiny Pokémon like that Poipole I’m unaware of.
I will edit that Bulbapedia list to add the Altaria, GS Shiny Ultra, and that Poipole after dinner. It also includes the Japanese exclusive Mew and XY-P Rayquaza, so I wouldn’t know why it shouldn’t also include all these others.
Greetz,
Quuador
@queendrifloon I’ve edited the list on Bulbapedia, so my txt-file from my Google Drive you can now ignore (and I will delete it again). 
For a more complete list, it looks like someone else on Bulabpedia already made such a list (except for two missing cards).
Greetz,
Quuador
Okay great! I’ll dig through it. I wonder how long ago it was completed; when I had looked on Bulba some months ago there were a lot of Japanese exclusives not listed, which is part of why I asked you. As I said before, you’re very thorough!
Well, it could be that I missed some. With my Pokémon lists, I use a specific method - which I’ve explained here: www.elitefourum.com/t/list-of-all-tcg-cards-that-exist-for-a-specific-pokemon/16709/163. With Shiny Pokémon I have to use a different method however, so I mainly used that first list on Bulbapedia as reference, and added some I knew were missing like the Altaria and GX Ultra Shiny set.
But since I missed that Poipole, I’m wouldn’t be surprised if there are more Japanese exclusive cards like that which I don’t know about.
Greetz,
Quuador
This has me in the researching mood (and I’m procrastinating a work project) so I’ll see what I can find for Japanese exclusive shiny variants.