Is this sentence "turn up" is used to refer to attendance. They didn't expect that so many people would attend. Side note: "turn up" is also a relatively new slang term used commonly in American hip-hop culture to describe how excited or enthusiastic someone is. So if you hear this in a rap song, they're probably not talking about attendance. ;)

In this situation, the meaning of "turn up" means to "show up" or "appear". Another sentence example: I lost my phone last night but I am sure it will turn up later.