If I play a spell like fireball that has an X in the mana cost, can I wait first to see if opponent counters it and only then decide what X will be?
If I play a spell like fireball that has an X in the mana cost, can I wait first to see if opponent counters it and only then decide what X will be?
no. you have to make all choices and pay all costs when you announce and it goes on the stack. They won't get priority to even try to counter it until after then.