How many people do you have to kill in the undead yard?

For the Undead Yard side quest, there are a total number of 279 monsters. 156 digimon on the first floor and 123 on the second. If you just keep fighting and not try to find hidden doors, you will never get the digivolution. Their are 2 hidden doors on the first floor and 1 in the second floor.

1 of the hidden doors in the first floor is in a T-junction near the NE of the entire place, and the other one is by the electric floors. On the second floor, fight those little digimon on the first arena and keep moving to the left but don't go off the legde yet. Go fight more digimon on the next arena.

Then you have to go back to the first arena and fight another wave of monsters. Now jump off the legde but do not destroy the "mummy creating machine." Head down and fight some Mummymon. When you get to the portal that sends you back to the DGS base, instead of sending yourself back, go down alittle more to the southern wall and destroy it.

You will reach another arena of those little digimon. Next head to the left to fight 3 Pharomon, 1 at a time though. If you have time keep heading to the left after the 3 Pharomon to recieve 5 X-data from a treasure chest.