All of them*.
Provided said clients have some that the agent is absolutely sure he or she can sell.
They don't need to seek clients out, however, because reputable agents get 100-500 (or more) queries a week. They have all the potential clients they can handle.
If an agent knocks on your door without you just having published a short story or what have you? Run. Fast.
*With the possible exception for agents for mega-authors like Steel or Grisham, who have enough on their plate without adding more clients to it.