It depends how many lessons you’ve got in the skill you chose to use :)
From page 51, Dice Stacking:
Take a number of dice, according to the skill chosen and the number of lessons beneath it:
- Stack three dice if you have fewer than three lessons under a skill.
- Stack two dice if you have three, four or five lessons under a skill.
- Stack just one die if you have six or more lessons under a skill.