Hi philco,
Unfortunately, there is no absolute answer to your question, as this depends so much on your individual setup and requirements.
As a general rule of thumb, please find below a quote from page 7 of the User Manual:
"Audio Preferences: You may change the number of used threads in the Multiprocessing drop-down menu in real-time, and each Vienna Ensemble PRO instance will use the specified number of process threads. So 1 instance on an 8 core computer should use 8 threads, 2 instances should use 4 threads, and so forth. Ideally you should have as few threads as possible. If you're also running your host sequencer on the same system as the running instances, it might be wise to reserve a core for it.
To keep it simple, set the amount of threads to your amount of cores. If you run into performance issues, lower the amount of threads.
A note on Multiprocessing
There are some general guidelines to follow when using Vienna Ensemble Pro on a system with multiple cores. The general rule to follow is that the optimal number of threads on a system should be equal to the number of virtual cores present.
Vienna Ensemble Pro, like most other hosts today, offers multi-threading. This means that it runs instruments and plug-ins in parallel using several different threads, which allows to utilize several cores on the system. Vienna Ensemble Pro Server will generally perform best when running as few instances as possible. With the VST3 or RTAS plugin, this is possible to achieve by increasing the number of midi ports per instance (see below). Using the AU or VST2 plugins, you might be required to run several instances to work around the 16-MIDI-channel limitation of these standards.
If you are running a sequencer on the same machine as Vienna Ensemble Pro, you could lower the number of threads for Vienna Ensemble Pro, to represent the estimated load of Vienna Ensemble Pro versus the load of your sequencer. In a system with 8 virtual cores and an estimated even load between sequencer and Vienna Ensemble Pro, a good starting point would be to set Vienna Ensemble Pro to 4 threads (if running a single instance). When running more than one instance of Vienna Ensemble Pro, the number of threads per instance should be lowered accordingly.
Certain plugins, such as Native Instruments Kontakt, also offer the possibility of multithreading, which may complicate things further. If using Vienna Ensemble Pro to host such a multitimbral plugin exclusively in one instance, it might give better performance to turn on multiprocessing in Kontakt while keeping a low thread count (1-2) for Vienna Ensemble Pro."
Hope that helps!
Best, Marnix