The base Rules (v2.0.0) say you may discard a Luck cube to reroll any number of dice once. I read this to mean that when you discard a Luck cube you get one reroll only. However, I could also read this to mean that you may discard Luck cube only once during a quest. During a single quest, may I discard a Luck cube, reroll and if I don't like the result, discard another Luck Cube and reroll again?

You can use Luck multiple times on a single Quest. You just have to discard the Luck cube each time you choose to use it.