+ 1 Coin per Coin token you've taken with Stockpile this game.
While this is in play, when you buy an Action card which you do not have in play, take a Coin token.
When you gain this, take a Stockpile mat, where you will keep the Coin tokens taken with Stockpile.
This is worth 1 Coin for each Coin token on your Stockpile mat. The tokens are not expended and remain on the mat.
Only buying Action cards gives you the Coin tokens, not gaining them in other ways, but if you can buy more than one Action a turn, or if you have multiple Stockpiles in play, you would gain multiple Coin tokens in a turn.
However, if you King's Court a Stockpile, despite having played the card three times, there is still only one copy of it in play, so it would only get you one Coin token.
You will never get a Coin token for buying another Stockpile, since it would be in play.