Hi Wei,
To get the actual value of the coupon discount is possible but will take a little effort.
To look at isolating a specific discount value, this doc might be able to give you some more insight into how that is possible:
https://docs.xperience.io/k11/e-commerce-features/configuring-your-store/checkout-process/displaying-shopping-cart-details-on-pages
Alternatively, you could write your own macro (which is what I would recommend if you want something very specific) which queries the order and retrieves the discount that was applied by the coupon.