The value of X is determined only as the undergrowth ability resolves. If the number of creature cards in your graveyard changes later in the turn, the target creature is unaffected.

Necrotic Wound's replacement effect will exile the target creature if it would die this turn for any reason, not just immediately after Necrotic Wound resolves.

If you have no creature cards in your graveyard, the target creature gets -0/-0, and it will still be exiled if it would die this turn.

Creature cards with other types, such as artifact creature cards, count for undergrowth abilities.

Because tokens aren't cards, they never count for undergrowth abilities.