There is a total of 84 trading cards to collect. These cards can be purchased from the shop, which can be accessed from the Main Menu > Game Progression > Shop. It's the last item in the shop. The costs are 250 points for 5 of them.
Points are earned by playing through the different modes and can be earned in abundance by meeting certain goals in the 'Title Defense Mode'. Like it is in real life, it is possible to buy cards you already have and the more cards you have, the bigger the chance is to get cards double, but just process through the modes to earn points and you will eventually complete your collection.