先行词前有many的定语从句一定要用that引导吗

不一定.而且只有当much作不定代词时才有这种用法.如果much表示数量（修饰不可数名词）,就没有这种规定.many一般不用作不定代词,只表示数量（修饰可数名词的复数）.所以也没有这种规定.如：
I have many books that were bought last year.
that指物时可以和which互换,指人时不可以和which互换,但可以和who互换.如：
There were many children on the playground who/that were playing games.

先行词前有many的定语从句不一定要用that引导。
在下列情况中，只能用 that，不用 which：
①当先行词是 all, a lot, (a) little, few, much, none, anything, something, everything, nothing 等词时。如：
All that can be done has been do...

先行词是不定代词much才要用that，这里much不是数量词，所以many没这个用法