6-8 display boxes is only if you want to chase legendaries. For casual players, 3 display boxes is good. With even distribution you end up with 2 of each rare, 2-3 of each uncommon, and 5-6 of each common. Since even distribution is unlikely, ideally you would balance your collection by trading with other casual players who have gotten into the game with the discount prices. Unfortunately, I haven't seen any trading.