So which of the legendaries/Mythicals are unique (Or in a known limited number) and which ones are merely super rare Pokemon?

So far, I think that the merely super stupidly rare pokemon are -
Kanto Trio
Elemental dogs
Heatran
Manaphy/Phione
Mew
Latios&Latias
Darkrai
Cresselia
Shaymin
Cosmog and Cosmoem
Solgaleo and Lunala
Nihilego
Buzzwole/Pheromosa
Xurkitree
Celesteela/Katarna
Guzzlord
Meloetta
However,
There are 2 Mewtwo
5 Genesect
There are like a handful of Deyoxys?

For the utterly unique mons...
Groudon/Primal Groudon
Kyogre/Primal Kyogre
Fug/Mega Fug
Lugia
Ho-Oh
Celebi
Jirachi
Lake Guardians
Dialga
Palkia
Girantina
Arceus
Genies of Healthy Meta
4 Muskedeers
Reshiram
Zekrom
Kyurem
Victini
Xerneas
Yvetal
Magerna
Volcanion
Zygarde (Assuming Zygarde is one huge disjointed conciousness spread over the world through cells and cores)
Necrozuma

Finally, the Regis are special because there's 1 Regigigas but he can create multiple Regi golems.

Is this right? Or are all legendary pokemon completely unique?