I was surprised by her words,which made me recognize what/how silly mistakes I had made

what
在这里 how+形容词
what+名词
后面有mistakes
所以用what

当然是what了.what后面跟名词,how后面跟形容词.

silly愚蠢的意思,silly mistakes愚蠢的错误,名词.

我吃惊于她说的话,让我意识到我犯了一个多么蠢的错误.

how，形容mistakes的程度

因为后面跟的是名词（silly mistakes），所以用 what
不用 how 是因为 how 后面只能跟形容词，如果用 how，上句应改为：
... recognize how silly the mistakes I had made.

what ,在这里做made的宾语

how