No player may take actions between the time the cards are chosen at random and the time they’re returned to your hand.

If X is greater than or equal to the number of cards in your graveyard, you simply return your entire graveyard to your hand.

All players get to see which cards you chose at random as they’re returned to your hand.

Reap the Past is still on the stack while you return cards to your hand, so it can’t return itself.