Usually when a hive swarms the old queen takes half of the colony with her and leaves half with the new queen. I'm not sure that happens every time but unless there was a problem in the hive that cause the swarm there will always be enough bees left behind to build the hive back up.
What you lose with a swarm is honey production because the swarm eats honey to fuel their move to a new home and if you catch that swarm they are busy settling in their new home instead of making extra honey.
