Get a cube, make it editable, move the axis to the bottom of it. Get a cloner object and make the cube a child. Set the cloner object to grid array and set how many building you want. Now with the cloner object highlighted get a random effector from the mograph menu, in the parameter tab of the random effector uncheck position and check ON scale.
Now you can randomize the height with the Y scale in the random effecter and control the amount of city with the cloner object.
/Randy