If I reveal Focused Offense with the text on The Tides of War can I do one more challange then or does the one I already did count?

I reckon it's like this:

You can only initiate one challenge while the plot is revealed. It doesn't say "you cannot initiate more than one challenge this round", and even then I doubt it would count the challenges you initiated while a different plot was revealed - what if you had initiated two challenges before revealing it? It couldn't magically undo one of those.

That sounds about right, thanks!

I have a question too about Focused Offense. The text says I cannot declare defenders, but I'm still able to use the "Horseback Archers" ability (Pay one influence: put into play knelt and attacking/defending, then at the end of the turn, if still in play, return to your hand)? I do not understnad If I implicitly declare them as defending...

You can still use the Archers. Using their ability is not declaring them as defenders. Same thing with CS Greatjon Umber and LoW Catelyn Stark.