18/8 stainless steel question

I understand that for stainless steel to be stainless it needs at least 11% chromium. I also understand that adding nickel is needed in order to produce
an austenitic sturcture at room temperature. But when the nickel is added - say 8% - why does the chromium need to be higher - say 18%? What would happen if 14 or 16% chromium were used instead?
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