When you see X on the card, it is taken as a variable cost. In most cases (like in The More the Scarier), it is the payment cost of the card.
As such, treat X as the amount in which you had paid for the card. However note that in the instance of this specific card, there is a hidden restriction in that you have to choose an X such that for the variable X you choose, there are sufficient legal targets. Should you choose an X which is illegal (insufficient amount of allies), you can't even put it on the chain.
In short, X is the amount you pay for the card and has the maximum limit based on the number of legal targets at the time you play the card.
eg: If you have 6 resources but only 4 valid targets for The More the Scarier, you may only pay 4 for the card thereby dealing 4 damage to the 4 targets for 4 resources.
*For choosing of X, you may also choose 0 for the number (to activate cards which triggers on abilities played).
Some people learn, some people don't. For me, I can never correct myself to say Julia Graves as opposed to Julia Roberts