Technically, what you would need to set the sample buffers to for glitch-free DAW operation would depend upon what is likely to be the worst instantaneous performance of any of the cores. Not easy to determine, which is why the ASIO Performance meter exists.
The more cores you have, the less likely any one core will be saturated, so for a given CPU clock rate (and instructions per clock cycle), more cores eases the burden.
However, I have noticed that Cubase does slightly favour one thread (which may be due to the GUI handling) and hence that may result in a tendency to push the ASIO meter more than if the CPU usage was completely even-handed.