Noob question about NM's....?

[ImortalKnife]

So far in this game I've run into 2 NM's(that I know of). How many times can you fight and kill a NM? Or is it once you have fought it and defeated it, it never appears again?
Thanx to anyone who can answer this.:)

[Spiritbear]

Some NMs have random pops, some are on a timer, and some have placeholders you have to kill to get them to pop. But you can kill then numerous times while you are playing this game.