Answer:
Three.
Step-by-step explanation:
An adverbial phrase is a group of words that modifies either an adjective, a verb or an adverb. They are used to provide more details about the adjective, adverb and verb in the sentence.
In the given sentence, there are three adverbial phrases that can be found. The three adverbial phrases are-
"covered in dust and webs"
"safely hidden"
"under the mildewing books and tattered old newspapers".