The correct answers are Many a dusty scroll has been deciphered in this laboratory. and A great number of dusty scrolls have been deciphered in this laboratory.

"Many a" is an advanced, slightly formal construction that takes a singular countable noun and a singular verb (e.g., Many a scroll has...).

"A great number of" correctly pairs with plural countable nouns and plural verbs (e.g., A great number of scrolls have...).

"Amount" is used for uncountable nouns (like water or courage), not countable nouns like scrolls.