The numbers you may choose include 1 and 10.

Effects that increase or reduce the cost to cast a spell don't affect that spell's mana value.

For spells with {X} in their mana costs, use the value chosen for X to determine if the spell's mana value is the chosen number. For example, if the chosen number is 4, a spell with mana cost {X}{R}{R} cast with X equal to 2 would cause Talion's last ability to trigger.