Only one hall can be designated for each settlement, but it can be re-designated with no penalty.
Settlers can be told to wait in the hall when they have nothing to do under the settler preferences (F4>Autonomy>Wait in Hall while idle). This is useful to stop settlers from hanging around dangerous areas far from your settlement.
Halls can be placed on any flat surface but cannot span more than one vertical level or exist on slopes. The Hall tool is found under Design>Hall.