1. Is there a maximum limit you can have with cards being in your hand?

2. If I were to attack lets say green based challenge and my opponent loses the challenge, but doesn't have any cards in his hard. Do you just not attack with that type of attack? Or attack and get a power coin to satisfy the claim? How does that work?

1. No. You can have your whole deck in your hand.

2. If I were to attack lets say green based challenge and my opponent loses the challenge, but doesn't have any cards in his hard. Do you just not attack with that type of attack? Or attack and get a power coin to satisfy the claim? How does that work?

Is this a strategy question or a rules question?

There is no rule prohibiting you from attacking a player with an Intrigue (green) challenge if they have no cards in hand.

If you win the challenge you will attempt to discard cards from their hand, but no cards will be successfully discarded. You will gain unopposed power as normal.

Why, strategically, might you decide to attack if they have no cards in hand? In the event you feel likely you could get the unopposed power.

Hope that answers the question.