Their meaning is close to each other but not exactly the same. Both indicate something surprising and unexpected but "coming out of the woodwork" indicates a multitude (many) and "came out of nowhere" usually means one occurance unless modified by a plural form of speech (they came out of nowhere). I have clients coming out of the woodwork. - A LOT of unexpected clients. The client came out of nowhere. - One client because the singular form "the" was used.

come out of the woodwork = come from everywhere So yeah, they are different.

"Come out of the wood work" likely compares the number of ___________ (often people) to bugs hiding in molding or in the walls. So yes, it is only used in situations where it can be hard to count the total.

They mean the same thing.. It means,' to appear suddenly and unexpectedly'