A number of the students is about 50 in my class.
參考答案:The number of the students is about 50 in my class .
a number of,the number of
a number of是大量的 等于a lot of一类 后面接名词复数
the number of是......的数量 后面接名词复数
例如:A number of apples are red
许多苹果是红色的
The number of students is 2000
学生的数量是2000人