Deck size is a interesting question in this game. 50 is the minimum and a "optimal" deck will be 50 or at least very close.. BUT card draw is insane in this game. I played a 3 payer game a week ago were we had 4 UC on beravor and bilbo in the table... that is 12 cards a turn... not including Brand's untap effect or mathoms... So potential for more.
When you have card draw like that running a large deck is not a problem, as you are drawing so many cards. This has lead to a deck set up for 2player that can be modified for 3+ were you have a streamlined attack and defense deck running 50 cards and the 3 copies of UC, Steward and Belt, you can also run 2x Steward and 2x belt if you wish, it just depends on how fast the decks need to be for the give quest, you might also want to run Protector in the streamlined deck. This deck targets the 2nd deck for all UC and the steward and the Belt. The reason these cards are in the 50 card deck is it ensures that these get drawn.
Now the 2nd deck is huge.. we are talking like 100 cards, you can get away with more even.. but a 100 cards is a good size I feel. You include your own copied 2x steward and Belt and UC as well to increase the draw. The idea is to load Beravore up with as many UCs cards as you can.. getting to a 6-8+ card draw is your goal. I think you can see where it goes form there.
This is a legit and VERY successful deck set up that works best with 100+ card decks...
My main point is that card draw in this game is crazy. If for example you are playing solo and running Bilbo (one of the BEST solo hero with out a doubt) you can easily go over he 50 card limit as you are drawing a extra card every turn. So while "50" is the what you should aim for.. feel free to go over it as there are many decks were this number is a loose number and some deck require way way more cards to function properly.
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