How do they keep track of the NPC zombies in Left 4 Dead?

I am talking about the NPCs that just walk into walls or wander around aimlessly. Even though the players cannot see them, they are there (say inside rooms or behind doors). Let's say there's about 10 or so zombies in a hallway and inside rooms. Does the game keep all of those zombies in a list and iterate through giving them commands? Do they just spawn when the user is within a certain radius or reached a special location?

Say you placed the 4 units (controlled by players) on completely different places throughout the map. Let's assume you aren't being swarmed and then you have not killed any of these aimless NPCs. Would the game be keeping track of 10 x 4 = 40 zombies in total?

Or is my understanding completely off?

The reason I ask is if I were to implement something similar on a mobile device, keeping track of 40 or more NPCs might not be such a great idea.