Hello,
At the Pirate Village in Map 3 close to General Merchant "2802,2310", you have 3 Treasure Chests:
#Bronze "1 Bronze Key Required to Open"
#Silver "1 Silver Key Required to Open"
#Gold "1 Gold Key required to Open"

In order to get 1 Silver Key, you need 10 Bronze Keys that need to be swapped by the NPC close to the Golden Treasure Chest "He has keys on his hands".
The Same thing goes for 1 Gold Key, you need 10 Silver Keys to be swapped for 1 gold Key.

With that said you need 100 Bronze keys to get 10 Silver Keys then get 1 Gold Key.

Hope it helps. Thanks