Now, I'm no expert, but I believe that it depends on your type of deck. Most decks need a reasonable amount of Trainers and Items, to get their Pokemon and for other utiliies. However, not all decks need a ton of energy. Check out trachy's Psychic Energy Recycle Deck: http://pokemondb.net/pokebase/rmt/13876/two-for-the-price-of-one-psychic-type-tcg-deck
As for Pokemon, you probably need a lot. Maybe 25ish?