<p>I'd really like to get into some D3D coding, but I don't have the time lately to learn C++ for what will amount to a hobby project.</p>

The number that your application needs; no more, and no less.
Obviously, if you're writing an application which contains some parallelisable algorithm, then you can probably start benchmarking to find a good balance in the number of threads, but bear in mind that hundreds of threads won't speed up any operation.
If your algorithm can't be parallelised, then no number of additional threads is going to help.