many修饰复数可数名词，表许多； much修饰不可数名词，表量或程度。
He has many friends, but few true ones.
There hasn't been much good weather recently.
（2）many a:
many a和many同义，但语气比较强，并且要与单数名词及单数形动词连用。
Many a prisoner has been set free. (=Many prisoners have been set free.)
〔3〕as many和so many均等于the same number of。 前有as, like时, 只用so many。
These are not all the books I have. These are as many more upstairs.
They worked like so many ants.
（4）as much等于the same amount of, 表同量和同一事情。
He bought two pounds of sugar and as much tea. 〔同量）
I was not in the least surprised, for I had fully expected as much. 〔同一事情）
（5）many和much之后不接名词时，作为代名词；另外much也可用副词。
Many of them were very tired.
I don't eat much for lunch. 〔代名词）
He is much taller than I. （副词）
参考资料：http://www.syzx.sdedu.net/xueke/english/new_page_many.htm