ok the basic problem of swarms in horror movies is that the growing evil is just having more of something, but past like 50 people can't tell the difference between orders of magnitude of things and so you need to show the swarm doing different stuff as it grows in size like showing emergent behavior by congealing into a superintelligent swarm or spawning a big version of the swarming entity.
they needed a BIG SPIDER or a BALL OF SPIDERS THAT BEHAVED AS A BIG SPIDER #monsterdon