A deck must contain a minimum of 39 cards plus 1 Hero card.
All of your cards must be of the same faction as your Hero.
You cannot have more than 3 copies of a card with the same name, regardless of rarity.
You cannot have more than 15 rare cards and more than 3 unique cards in your deck.
There is no minimum to the number of Rare or Unique cards you can add in a deck.