Creatures with variable power and toughness
Yes, that creature's power and toughness will change as the number of creatures in your graveyard changes. It doesn't say something weird like "when it enters the battlefield, count the creatures in your graveyard, etc etc". It just says:


  Boneyard Wurm's power and toughness are each equal to the number of creature cards in your graveyard.


So that's what it does. However many creature cards are in your graveyard right at this moment, that's its power and toughness. Add one, they go up by one, remove one, they go down by one.