My current project is: De_controlroom has monstermaker entities that spawn infinite amounts of monsters and will stop spawning once you press the button again. I you download it, it will answer you question. But its currently 75% done so.. You will have to wait for a while...
But if you want to know now:
Make a entity: scroll donw and choose "monstermaker" and name it
"monster". Scroll down and you will find (monster type), (children's name), (number of monsters), (frequency) and (max live children).
1. monster type: what is the monster that is being spawned? ex. monster_ human_grunt
2. children's name: dont type anything here because it just names the monsters. (literally)
3. number of monsters: monster stock. If you want infinite type
99999999999999999 on it. (Dont worry there will be no glitch)
4. frequency: how many monster pop out at a time. (I think)
5. max live children: the name sez it all.
Make a button in you map. Right click and Press "tie to entity" then choose func_button. name your button anything. Then on the trigger part, type the name of the monstermaker. On sounds, choose anything you like that suits your taste. leavet the "delay before reset" thing.
On "delay before trigger" type 3. Then, go to the Flags tab. And check boxes: Dont move and Toggle.
AND THERE YOUR DONE!!
P.S. I posted this here cuz I dont know how to post at the tutorials (if i can.)