a lot of 是既可以修饰可数名词，也可以修饰不可数名词。many只能修饰可数名词的复数。而且a lot of 可以修饰比较级，而many不可以。

例句：a lot of：

1、I have lost a lot of weight.

我体重减了不少。

many：

1、How many years have you been here?

你在这里有多少年了？