For example, you have 4 Ash Eater, 2 Photon, 1 Miracle, and 3 Fire Shield in your deck. This means that you have 4 types of cards, as you have 2x creature (Photon and Ash Eater, 1x spell (Miracle), and 1x permanent (Fire Shield). You may have any amount of the singular categories, so long as you have a total of 4.