I declared a simple number variable in the scene and left it empty (0) - Zombie_Spawn.
In the events, I start with a condition that no Zombie objects are in the scene and change the variable to add +1.
Then I start spawning one Zombie at a time in the scene, but repeat the event +1 to the variable.
And then it starts looping. If I kill all Zombie objects, there are 0 objects in the scene, so it adds +1 to the variable and repeats the creation +1.
Depending on where I place the “Change the variable” action, it will start the scene with 2 Zombies or with 1 - see the commented events in the screenshot.