It is a good question, with a lot of possible answers. So many variables come in to play, such as: Library, how many instruments, all concurrent playback, daw, sample rate, buffer settings, etc.
It is pretty hard to have a guess, based on the info you provided. If you share more of what you are trying to do, there may be some folks here who can chime in and give a starting point.
So much will just depend on your workflow and what you are trying to accomplish. Best advice is probably just experiment.
At the moment, I am using 3 instances on my local host. Each instance gets 3 cores. I have preferences set with 8 midi ports (128midi channels) 256 outputs and 64 inputs on each instance. I am not even using any slave machines at this time.
I have a 10core CPU without hyperthreading enabled. I leave one core for DAW. I record at 48khz sample rate, 24bit, with a 256 buffer. Each instance VEP connection is running with 2 buffers.
This works for me, for how I work. But, I am not doing giant orchestral stuff. I have 256 instrument slots setup, and then I use one instance for pure Send effects. It keep CPU hog plugins from getting in the way of my DAW.
All that said, I am still learning new ways to use VEP by listening to others and just trying things out. That is the beauty of it all...you can make it your own rig, exactly how you like to work.
Strings Complete Bundle (Full) - Woodwinds Bundle (Std) - Recorders (Std) - Vienna Imperial (Std) - Vienna Instruments Pro 2 -Vienna Ensemble Pro 6
Cubase Pro 9 - Waves Mercury - MeldaProduction MTotalBundle - Eventide AnthologyX Suite - Slate Digital Everything - Wavelab Pro 9
DAW VEP Master = Windows 10 Pro - i7-6950x (10core) - 128GB Ram
Dell P4317Q & UP3216Q 4K monitors
RME HDSPe RayDat - (2) Steinberg MR816CSX