How do you make enemies spawn?

I want to make a fortress style type map with some allys on your team
with some guns and you and your allies fight off hordes of
zombies. How do you make a spawn that will spawn about 1 enemy
per 2 seconds until there is say a max of 40 zombies out at once.
Then it stops and when ones killed another is spawned. How do i
make this and control it? and time it when to begin

NPC Maker

Be sure to look at how I set it up and then clone it. You'll want five or so npc_makers, each with different settings equalling up to 40 npcs. You don't want just one as there is a delay, and it looks odd.

Link for the NPC Maker is gone :sad: