How does "多" work in this describing sentence "多好看啊!"? I know 多 means many. But in some cases, it is used as in this sentence, such as this one: "多好看啊!" How exactly does it work? Thanks!
Chinese words or characters are best not to be thought of as having specific meanings but rather of having a concept or a general meaning.

So in the case of 多 anything that roughly means many, a lot, very (like Rui said) can work