Its hard to follow you. You probably mean something other than what you've written.
how can i find the concentration of KOH which can produce
OK, for electrolysis, you'll get different results with widely varying concentrations. I can follow that. 10 ppm KOH, versus 50% KOH will give a different result, for sure.
maximum concentration of hydrogen gas
So different concentrations of KOH give different mixtures of H2
gas, some high or some low? Mixtures of what and hydrogen? I can't follow. You probably mean yield, as in amount.
if potential difference of 12v is applied supplying current of 18 Ampere ??
Yes. There is likely a formula that relates yield to volts and amps. I don't know it off hand, but someone here may be able to help.
if we are using 316L stainless steel electrodes give me just equation or reference to find this... i shall be very thankful.
Sure. But ... you seem to miss some of the basics. And I feel this will be an annoying thread if we don't get the basics clear. Do you know the chemical reaction for hydrogen production by electrolysis? You really should start there.