"Each player may initiate an additional M challenge this round."

If I use this effect (i.e. attack twice with military) and I repeat the challenge phase with Den of the Wolf, then I can't attack twice again, right? I mean it's only 1 additional attack per round?

I believe you are correct, Rogue. Even though you are repeating the phase, the plot still specifies *round*. So if you already used the additional challenge during the first challenge phase, it would not be available in the second, since you are still in the same round.