The value of X is determined only as Fraying Sanity's triggered ability resolves. For example, if three Fraying Sanity Auras are attached to one player who had four cards put into their graveyard this turn, X will be four for the first ability to resolve, eight for the second, and sixteen for the third.

Fraying Sanity's triggered ability counts the number of cards that were put into the enchanted player's graveyard during the turn, even if Fraying Sanity wasn't on the battlefield at the time those cards were put there, and even if those cards have left that graveyard.