IMO : E

Paintings = Countable noun
Number of paintings is correct.
So we are down to answer choice C,D and E
C- Subject -verb agreement.

Difference between D and E is just of a word "a museum"
Original sentence talks about single museum and E maintains originality.

IMO E.

The amount of paintings is wrong. Eliminate A, B.
The number is singular - Eliminate C.

Between D/E the article 'a' is necessary.

OA is E

C is not selected for this reason....
While this answer choice corrects the original mistake, it creates another mistake by replacing the singular verb is with the plural verb are. This creates a Subject Verb Agreement mistake with the singular subject The number of paintings.Two Stop Signs help us identify this mistake:An X of Y subjectA long and complex subject / a long distance between the subject and the verbThe lesson to be learned here is not to jump at the first answer choice that corrects the original mistake. After eliminating the two answer choices that repeat the original mistake (that is, (A) and (B)), you need to compare the remaining answer choices carefully and eliminate all but one of them.Remember: we do not choose a correct answer choice - we eliminate four incorrect answer choices.