How many relics do you need to hatch a dragon?

You need 15 relics to hatch a dragon, which matches the 1st star rank (1★).

Relics drop as "eggs" of 5 — so 3 eggs = 15 relics = one hatch. After that, raising the Star Rank costs more and more relics: 2★ = 40, 3★ = 100, 4★ = 180, and it climbs up to 7★ = 800 (the 5★/6★ tiers aren't confirmed yet).

Relics are permanent (cross-reign capital), just like Relic Shards. Tip: when you get relics for a dragon already at max Star Rank, they convert into Relic Shards, which you can trade for other relics or for keys.