The treasure covered in dust and webs waited beneath the stairs, safely hidden under the mildewing books and tattered old newspapers. How many adverbial phrases are there

Adverbial phrases in quotes.

The treasure "covered in dust and webs" waited beneath the stairs, safely hidden under the "mildewing" books and "tattered old" newspapers.

So, three.

Here are three adverbial phrases. The first is "covered in dust and webs." The phrase "waited beneath the stairs" is actually the predicate of the sentence, so don't count it as an adverbial phrase. The second adverbial phrase is "safely hidden." The third adverbial phrase is "under the mildewing books and tattered newspapers."