The reason why you cannot keep induction stove indefinitely on the highest power setting usually is that the cooling solution of internal electronics is not designed to reliably dissipate the maximum power loss continuously (it is similar idea as turboboost of current intel CPUs, some stove vendors even market it as turbosomething(TM)). As for maximum power most european (ie. 230V) induction stoves are designed to be powered from two distinct AC phases and the electronics can distinguish between two distinct phases and both power inputs shorted together and limit maximum total power accordingly.