How do I get the AI in my map to spawn once (like a campaign map)

You have to learn some simple Halo script code and then you can do that (ai_place)

If you are tyring to make an ai multiplayer map all you have to do is on the properties for the encounter just don't check the box that says respawn enabled, then when the ai dies it will not respawn.

Then the AI won't spawn at all...

Yes, you should uncheck the "Respawn Enabled" box in the encounter properties, but after you do that you need to compile a script telling Halo to spawn the encounter at startup (or any other time you want the encounter(s) to spawn), otherwise you won't have any AI spawning.

Here's a script example. The encounter name is "red_team" and the script is called "spawner". After script and map compilation the script should tell Halo to spawn the encounter when you start the map.

(script startup spawner
          (ai_place red_team)
)

You could also just fill in the "respawn total" box.