The number of, or a number of? This is not a minor detail.

A number of students *are* late. The number of students *is* small. The verb changes depending on whether the subject is "a number" (plural, meaning several) or "the number" (singular, meaning the count itself). This is a Band 7+ grammar point that Korean speakers often miss because of L1 transfer. Get it wrong in an IELTS essay and it directly hits your Grammatical Range and Accuracy score.