how do i et npcs to keep spawning?

i found cool maps with never ending zombie npcs i want to know how to keep them spawning

They’re just zombie spawns set to timers. I think there’s an entity out there(really old) that allows you to make NPC spawners that can be set to timers.

You can use this addon (Same thing just with EP2 NPC’s too) http://www.garrysmod.org/img/?t=dll&id=45003