Should we repeat after all muezzins, or is it sufficient to respond to just one muezzin if more than one muezzin calls the Adhan at the same time?

Responding to the Adhan (call to prayer) is a confirmed Sunnah. If a Muslim hears more than one muezzin, the prescribed action is to respond to every muezzin he hears, as long as their Adhan is legitimate. If the voices of the muezzins overlap, it is best to respond to all of them if possible; otherwise, one should respond to as many as he is able to follow.