The Keystone Forge Card says:

Keystone Forge - Forced: After your turn begins, heal 1 damage to your capital.

Does this mean every time my turn begins i can heal 1 damage? or only once after having played the card.

As I read the rulebook it only happens once because its not an constant effect.

Ift it happens all the time it would be quite powerfull, wouldn't it?

It is a forced effect - so that means every time the condition is met - in this case the condition is "after your turn begins" - then you perform the action - heal a damage.

This card also stacks, so if you have 3 in play, then every turn you heal 3 damage from your capital.

It is a good card, yes

Page 15, "Forced effects are triggered by specific occurrences throughout a game, and they occur automatically, whether the card’s controller wants them to resolve or not. Forced effects always occur immediately whenever their trigger is met, and they cannot be cancelled or interrupted by other actions."

According to the rules under Forced Effects they trigger every time the conditions are met throughout the game.