Many responses included the answer 4 for any type of Pokemon unless further restricted. One year, many many moons ago, Sneasel, a dark Pokemon, was banned in any deck. The official Pokemon web site publishes the rules for deck building and playing, but an event may also have additional rules and restrictions.
Of course energy is one of the card types that is not restricted, unless it is a special energy. Just for fun I used to make a psychic deck with mostly psychic energy and just 4 pokemon! Because I get to shuffle untill I draw at least one basic pokemon in my opening hand the deck was annoying, but I always had plenty of energy to fuel my clever deck.
So to be a good "Pokemon Trainer" when you play the card game you need to use the right balance of trainer cards in order to draw cards and get to your key evolutions, key basic pokemon, and trainer cards. If you go online you can find the four top decks in the senior division and the junior division of the world championships. You will notice a common ratio of Pokemon, trainers, and energy in these top decks. Try to imitate their ratios and even their decks, especially how and why they use certain trainers and you will be competitive in a local Pokemon league or against your friends. Notice which basics they use that do not evolve, and notice the weakness and resistance they have. Pokemon is a "don't put all your eggs into one basket" type of game, influenced by who gains control first with a key knockout, and by the ability to play an energy every turn or more. (Blastoise was so powerful because of his ability!)