If you want to limit the number of existing enemies on the layout (maximum 100 for example), add this condition to your event where you do the spawning:
System compare two values Enemy.Count<100
.
If you want to limit the total number of spawned enemies (including the ones that are already destroyed etc.), you need to add a variable SpawnCounter. Add 1 to it every time you spawn a new enemy. And before spawning check if SpawnCounter<100