1.a number of只能修饰可数名词的复数形式,意为许多...谓语通常用复数.如：
A number of students were absent from the meeting.
许多学生没有到会.
经常会与之混淆的是the number of的用法.
2.the number of后面也是接名词复数,意为...的总数,谓语通常用单数.如：
The number of students absent from the meeting was surprising.
没有到会的学生人数很惊人.