Hi BWT,
using more than 4 parchments wouldn't change anything to the mechanics of the game/battle. The parchments are masked before, so there is a chance for both low and high numbers. As a beginner in this game, Legend 3 should be quiet easy - but if you like it the harder way, you can pick the highest numbers and play again
As far as i remeber there is at least one fate card in the beginning of the game, which requires a parchment, too. Once the fate is fullfilled, this parchment is out of the game. Thats why you cant use all 6 for the boss.
greetings
PS: When the fatecard is done, there is no need for the first parchment anymore. So in principle it could be used again in the later game - but not in Andor.

(what's done, is done.)