"so many ...that"

Solution 1:

The example sentence has a very close semantic realtionship to the following:

There were many webs, so that the town looked like something from a horror movie.

In this sentence, so is the head of a prepositional phrase functioning as an adjunct of result.

In the original sentence so is an adverb of degree modifying many. It licenses the post head content clause following webs which expresses the result of there being so many webs. The net effect is thus essentially the same.

We can see that there is a difference between so as head of a prepositional phrase and adverb as we can drop the subordinate clause that the town looked like... in the original, but not in the version above.

There were so many webs.

* There were many webs, so.

ref: CaGEL p734