Most of them except for some few like Smaug and Scatha were killed in the War of Wrath at the end of the First Age (about 6500 years before the War of the Ring) when Morgoth was defeated... the others remained hidden in their dwellings most of the time...
The only two dragons that are mentioned in the Third Age by their name are Scatha and of course Smaug... and both only because of their death 😬
Scatha was killed by some Rohirric king or king's son... I think it was Fram...
And Smaug, well, Bard of Dale killed him 😬
I think there are also only two dragons called by name in the ages before, Glaurung the Golden and Ancalagon the Black, Father of Dragons... two of the most amazing creatures Tolkien ever invented, especially Glaurung 🙂
damn, Exa best me to it. As she said, 98%of the dragon are dead. Any remaining are weak and small and not legendary lkike the other onces.
Anyway, more on Scatha
One of the greatest of the dragons to infest the Grey Mountains of the north. Of his life little is known, except that he was slain by Fram son of Frumgar (an ancestor of Eorl the Young) in the early days of the Éothéod.
His recovered hoard was the subject of great dispute between the Men of the Éothéod and the Dwarves of that region, who claimed the hoard as their own. Fram rebuked their claim, and sent them the teeth of the dragon, with the words, "Jewels such as these you will not match in your treasuries, for they are hard to come by."2 Thereafter there was war between the Éothéod and the Dwarves, a war in which Fram met his end.
However this dispute was resolved (we are not told), it seems that Fram's people retained at least some of the hoard, and brought it south with them when they settled in Rohan. The horn that Éowyn gave to Merry Brandybuck after the War of the Ring (many hundred years later) was said to have come from it.