You should be aware that applying voltage to two pieces of metal in an electrically conductive solution of water will produce HHO.
The negative electrode will produce the Hydrogen, and the postive will produce Oxygen.
Electrolysis will begin to occur at around 1.67 volts DC. Most supply voltages are based on current available battery voltages.
Let's use 12 volts.
By applying that voltage to only 2 plates, the voltage between those plates will be equal to the total input voltage. The problem with this setup is that 12 volts is too high. You want to maintain between 2 to 2.4 volts. This minimizes wasted energy in the form of heat.
In order to reduce the 12 volt supply voltage to the desired 2 to 2.4 volts you need to add additional plates that are not electrically connected. These are called neutral plates.