a number of不可以修饰不可数名词,它修饰可数名词
number可数,所以a number of就修饰可数名词
可以用a huge amount of/an amount of修饰不可数名词
the number of是……的数量,和它意思大不相同
We have lived here quite a number of years.
我们在这儿住了好些年了。
A number of problems marred the smooth running of this event.
许多问题影响了这件事的顺利进行。