They usually take it from open cells unless all those are empty. Then they open whatever is near where the open cells were which could be the top super or the near the brood nest. I don't know of a for sure number, but they aren't hauling nectar, so it's pretty concentrated and it's all they can carry. Obviously the honey can't weigh more than the bees, so a five pound swarm must not have more than a pound or two of honey in them.