Once you have reached max level for any card in your collection, where Common Card max level is 26, Rare Card is level 24, Epic Card is level 22 and Legendary Card is level 20 you've hit the maximum amount of that card that you can get.

From that point onwards, when you would receive more copies of that card from a Pack or from a Safe House, you'll receive Coins instead since you can't collect any more of that card.