You can collect hundred maximum cards. Wait until you reach that limit before upgrading your cards. This allows you to determine which cards will be kept, and which will be used as upgrade fodder. If you have doubles you can combine them to create a Pro card and improve its stats even more. However, do not combine two identical cards until each of them has been maxed out in order to recieve its full potential. Note: You will lose your enhancements when cards are combined; wait to Pros to max them.