Hi there - and thanks! Let me clarify:
1) Anyone can roll up to their Hearts in dice - max three, minimum 1.
2) If someone loses a Heart, they lose one die; if you've got 2 Hearts, you can only roll 2 dice.
3) Therefore, you still have a ton of Lines to choose from ... but you can only use as many as you have Hearts.
The idea is to create a mechanic that makes people think about how their character would, in the fiction, suceed. While there might be a lot of Lines one could leverage, they won't always apply in the fiction; sure, you might be besties with Rex the dragon, but if Rex isn't in the scene, getting a dice for that doesn't make much sense. Everything depends on how the player describes why their character will succeed; if they want to put a dragon to sleep, a character with the line "incredible voice" might sing, where a character with "dumb luck" might get half-eaten before the sleep potion in their pocket from earlier glugs down the dragon's gullet (and they after have to struggle out of the thing's mouth as it snores!).
When a line's crossed out, it's just not available to use! This can be especially handy if a player's been leaning on one line too often; removing it pushes them to think about the other ways in which their character matters.
Hope that cleared it up - and let me know if it didn't!