Per unit, as mentioned

.
But I do like the idea of only certain zombies breaking in, could imagine some falling through windows or breaking through doors etc, while the rest mumble around outside.
The bonus of per unit, is that while you may not every single unit in, you could get a decent number of zombies inside the building. The downside is, depending on dice rolls, it may take some time.
With 6+ per model, you're guranteed almost to get zombies inside. On the downside, it maybe not a huge amount and may get confusing which zombie is from which unit.