I am interested in making a card game for the sake of passing the time and I notice that in several games such as the World of Warcraft TCG and Magic: the Gathering, only four copies of a card are allowed. I know the idea of restricting overflow of one type of card plays a part in this, but why is the number four? I think it’s 3 in the Yu-Gi-Oh! game but I’m not sure.

Just wondering if anyone could offer insight into this. Thanks!