You can have as many neopets as you want, in fact I believe you have to have the minimum of 40 cards, and then a stack of 10 basic neopets. Heros, Villians, and Experienced Neopets don't count as one of these 10, they count as part of the 40. To play a hero/villan/experienced neopet, you have to follow the directions on the card. Usually, this means you have to have a neopet with the same base type as your hero/villan/experienced neopet, and tap that basic neopet to put the better one into play. Each one has its own rules, though. Some of them require you to tap two cards as well. I don't know much about this game, but I do know that much
~Maria