You can change the numeral inside of a mana symbol, but you can’t modify the cost of a spell in time to pay less for it. It must already be on the stack to be a valid target for Truss’s ability.

Indefinite articles such as “a” in “draw a card” aren’t number words and can’t be modified.

You can’t use Truss’s last ability to change the 1 in Truss’s last ability to a 0. Nice try though.

You can’t change any instance of X.

Note that you’re changing numbers, not digits. For example, if you remove two hack counters from Truss, you could change 10 to 12 or 8. You couldn’t change it to 30.

You can change the number of dice rolled for an effect only if the original instruction included a number word to describe how many dice to roll. You can’t change the “a” in “roll a six-sided die,” but you could change the “two” in “roll two six-sided dice.”

Truss’s last ability affects a single number or number word on the spell or permanent. It can be anything—power, toughness, something in rules text, the name of a card (Four Wishes, anyone?), collector number, and so on. Mark says you can even change the copyright date, but don’t tell the lawyers.

You can change the number of sides on a die. If you don’t have any thirteen-sided dice handy, you can use other dice to simulate the rolls.