The key to building a winning deck is to have it balanced with cards, so that you will be able to draw what you need when you need it. Thus you need a good balance between energy, trainer, basic Pokemon, and evolution cards. A normal full deck has 60 cards.
1. The most important cards you should be thinking about is energy cards. Personally I think a deck should always have between 20 and 22 energy cards. The kind you choose should be based on what type of energy your Pokemon you are using in your deck need for their attacks. Try to build your deck so that you only need two or three different types of energy so you are better able to have the kind of energy when you need it. I find it useful to also make one of my energy cards be a colorless energy, which can come in handy if I have colorless Pokemon or I just need any type of energy to do an attack. Since you can only play one energy card per turn it is best that if you have one that makes sense to put on your active or benched Pokemon then do so each turn. Do not do so, however, when the card has the max energy it needs and it will not be evolved to need more. Also, try not to waste a specific type of energy that does not go with the particular Pokemon’s other attacks just to do a colorless energy attack unless you do not have any other choice.
2. Secondly you need Pokemon cards. Some may say this is the most important, as you cannot play the game without having Pokemon in your deck. While that is true when it comes to winning the game it can really come down to if you have the energy cards you need to use the Pokemon’s attacks. Depending on your use of trainer cards this part of your deck may range from 20 to 40 cards. Of the Pokemon cards the most important is having basic Pokemon in good supply that have decent attacks, as without them you may not last long enough to draw the evolution card and make it useful. A good strategy for Pokemon you have the Stage 1 and Stage 2 cards of is to have 3 of the basic Pokemon, 2 of its stage 1 evolution, and 1 of its stage 2 evolution. Also, remember when choosing your Pokemon to make sure that they all only require a total of two or three different types of energy (colorless does not count as a type when counting this, as it can be satisfied with any type of energy) and that those energy cards are in your deck.
3. Lastly trainer cards can be of use, although I do not find them to be entirely necessary. Personally the only ones I find useful are the ones that let you draw cards (do not overuse these or you may end up losing by using up your deck), heal/potion cards, and search cards (especially ones that let you search for energy). Depending on your card choices this part of your deck may be from 0 to 20.
Based on my deck building strategies described above the normal deck balance that I play with is 20 energy cards, 30 Pokemon cards, and 10 trainer cards. It of course does vary a little depending on the cards I choose for a deck, but the one thing I never do is go under 20 energy cards.
