System ZombieNumber=0 ---->Add 1 to WaveNumber
---->Set Zombienumber to Wavenumber
System Zombienumber>0
System-->Pick random spawner
Spawner ---->Spawn Zombie
System ---->Subtract 1 from Zombienumber
I *think* this would work. Not sure if you want that "trigger once" at all, let alone as a sub-event.