How to let a group of units spawn together via Trigger?

Can someone explain to me which triggers I need to activate to spawn a group of units on the map under a certain condition?

I have the idea of spawning, say, 20 Monaspas from Player 3 as reinforcements on the map when I declare war on their ally, Player 2.

So far, I’ve only managed to spawn a few Monaspas via timer and trigger.