There are a total of 9 legendary armaments in the game.
Below are links to each of their wiki pages, beware spoilers if you click, but these will have exact locations for each of them:
1. Ruins Greatsword:
https://eldenring.wiki.fextralife.com/Ruins+Greatsword2. Eclipse Shotel:
https://eldenring.wiki.fextralife.com/Eclipse+Shotel3. Grafted Blade Greatsword:
https://eldenring.wiki.fextralife.com/Grafted+Blade+Greatswo...4. Sword of Night and Flame:
https://eldenring.wiki.fextralife.com/Sword+of+Night+and+Fla...5. Marais Executioner's Sword:
https://eldenring.wiki.fextralife.com/Marais+Executioner%27s...6. Dark Moon Greatsword:
https://eldenring.wiki.fextralife.com/Dark+Moon+Greatsword7. Devourer's Scepter:
https://eldenring.wiki.fextralife.com/Devourer%27s+Scepter8. Golden Order Greatsword:
https://eldenring.wiki.fextralife.com/Golden+Order+Greatswor...9. Bolt of Gransax:
https://eldenring.wiki.fextralife.com/Bolt+of+GransaxNote: This achievement is missable, and if you advance the story past a certain point you will not be able to get this without starting the game over.
Major spoilers ahead, read at your own risk.
This is just to explain which weapon is missable, and what causes it to be missed.
*** Spoiler - click to reveal ***The weapon 'Bolt of Gransax' is missable, as the location to get it will be buried under ash once you defeat 'Maliketh, The Black Blade' and will no longer be accessible.