The value of X is calculated continuously as the number of creature cards in graveyards changes.

If there are no creature cards in any graveyard when Bonehoard's living weapon ability resolves, the Germ will be 0/0 and put into its owner's graveyard.

Although creature tokens go to the graveyard before ceasing to exist, they never count as creature cards and won't increase the bonus granted by Bonehoard, however briefly.

If lethal damage is dealt simultaneously to the equipped creature and another creature, both creatures are put into the graveyard at the same time. The equipped creature won't get an additional bonus from the other creature.