If you are subtracting health on collision event, then it's very unlikely that 50 zombies will collide with the wall all at the same tick (1/60 of second).
But yes, it is still possible that more than 1 instance will collide, so you need For each loop:
Zombie On collision with Wall
System For each Zombie -> Wall subtract 5 from health
Or use the PickedCount expression:
Zombie On collision with Wall -> Wall subtract (5*Zombie.PickedCount) from health