Well, really it's like InflatableMouse said, the more the better. However, in practical terms, we usually optimize the defaults for various threaded operations to make good use of a 4-core cpu (8 including hyper-threading) since that is pretty common these days (e.g. 4 core i5 and i7). Audio playback does use several threads, but the most intensive threaded operations are things like media import, conversions, audio analysis, library operations, etc.
MC will use multiple threads whether the system has a single core or eight cores. Of course it will run faster if the OS can divvy up the threads to separate cores rather than running all on one.
You can use Activity Monitor to see how the cores are being used during playback of your DSD files.