so at most, a deck would require 60 * 5 = 300 bytes... if every card were unique. So thats still quite small.
Even with 50 decks per player, 50*300b -> 15 kb per player
So for every 1.5 Gb of storage space server side, you could house decks for about 100,000 players, even at worst case of 60 card decks with all unique cards.