Anyone inside or touching the trigger is considered on the hill. You CAN make multiple brush hills if your editor allows you to make multiple brush triggers. It also allows you to add new KOTH starting points to a level you might have already designed, without having to go to the trouble of changing it significantly. This is normal. It will not effect your level at all.
Most map editors use QuakeED style header parsing to define what entities do what.
The brush to which this trigger is attached becomes an invisible bounding box. If you don't know how to use these QuakeED headers, check the web page for your map editor. A hill doesn't have to be a traditional bump in the ground. It should be FUN to get to the hill and FUN to stay there. Second, remember time considerations.