How many

[SkyeGU]

There are the Grunts that spawn after specific events (ie: entering the Kitchen, walking down one of the hallways (near the exit door), using the acid on the weak padlock and then breaking it(!!!), etc) but i'm fairly certain that there is also a Grunt that constantly roams the hallways. Other than those, there is a scene where a Brute(If I remember right) is seen walking past a doorway at the far end of a hall that freaks you out, but I don't think he ever strays from that. Then there is, of course, the room with the monsters in it that kill something but never come out.

So if you count the ones out and about there are at least one but at most two depending on what you've triggered. We shouldn't count the ones that can't get out.