If X is greater than the number of cards in the targeted player’s library, you can’t choose to have them mill any cards. This is because if you are given a choice and one of the options is impossible, you can't choose the impossible option. (That means you should make sure to cast Our Market Research Shows That Players Like Really Long Card Names So We Made this Card to Have the Absolute Longest Card Name Ever Elemental before their library gets too small.)
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.