[MaxWilson]

If you have 5 priests cast, they will try to spread out so they don't cover the same guys. I.e. casting Sermon of Courage twice on the same guys does nothing, so the 2nd and subsequent casters will try to target someone new. As Taqwus says, they may or may not have the range (or Precision) to hit everybody you'd like them so. Check the manual or the in-game description.

Edit: while we're on the topic of buffs, I'll add that mages like to target big, tough things. A mage next to an elephant or an Umbral, scripted to cast Iron Warriors or Body Ethereal, will usually cast it on the elephant or Umbral instead of himself. That's a good thing.

-Max