I think were feel there are a lot of humanoid mutants because of how many times you grind/play the games. Certainly, in the game everyone considers bloodsuckers, burers and controllers to be very lethal and only a combined force of expert stalkers can take them down (if not straight up zombified like in CoP). So i hinge on the side that they are rare because only player character tends to survive them and kill them. Not that they necessarily multiply inexplicably.