What RMS voltage is required for the polarization process?
I will tell you. Perhaps you remember those who do electrolysis with electrolyte and neutral plates. They put enough neutral plates between the positive and negative to divide the voltage enough to still cause electrolysis but keep voltage down to reduce heat. I believe, without checking, that it ended up being about 1.47V minimum to achieve electrolysis between two electrodes.
The same principle applies to the cells in series. The number of cells times the 1.47V (or whatever it is) should dictate the minimum RMS voltage to polarize the water and eventually create the gas. The only difference is that we are doing it with a resonant wave form and opposing coils to restrict the current. We then will use high voltage pulses rather than current to do the work. Higher voltages will perform more work, but my theory says the minimum RMS voltage will be based on the number of cells you have in series.