Yes, crunch is multi-threaded application.
If you want to know more about crunch, there is a source code inside a package, open it and see how many threads crunch creates.
However if your processor has 4 cores, that does not mean that all 4 cores will be used by the application at the same time (even if application created 4 threads).
Also note that an multi-threaded application is not the only one that lunches threads, there are usually hundredths of threads (of various applications) managed by the OS.
How many cores will be used by an multi-threaded application depends on 2 things:
1. How many threads does application creates.
2. How does system manage threads in a given time
I asked a simple question once on stackoverflow Have a look
Hope this answers your question