Maxperson
Morkus from Orkus
The books didn't state that one way or the other, but I think that they were not truly endless. My feeling is that the books assume that there are enough of them present that for the duration most groups would be in the area/dungeon, all rolls of a particular creature are valid and encountered.In my experience, which when it comes to random encounters is based on the B/X and AD&D rulebooks, wandering monsters tend to be regarded as coming from what is - for game play purposes - an endless supply of such beings.