Although it is grammatically correct to say 'the number is twenty', because the noun ('number') is singular, native speakers would be unlikely to say it in this way. It would normally be expressed as 'there are twenty students in her class' ('are' because the subject noun is 'students', which is plural).

Hi Anja. Cambridge David is absolutely right. While the sentence 'the number of students in her class is twenty' is grammatically correct, it is not a construction that a native speaker would ever be likely to use, in either a spoken or written context. It's not a question of formality, but a question of appropriateness. For example, if I wrote ‘2023 is the year’, I am grammatically correct, but this construction would instantly mark the speaker as a non-native. That’s the point Cambridge David was making. I hope you understand.

Guess the correct answer. (formal English)