How many processes can Windows support at the same time? This question is nothing to do with parellel processing of any sort. Processes can be running, waiting in the queue, sleeping etc,
There is an article: Pushing the Limits of Windows: Processes and Threads written by Mark Russinovich that explains everything relating to the maximum number of processes you can run in Windows. It's quite a fascinating read if you're actually interested in learning about the inner workings of Windows.