Your type of deck should dictate how many Pokemon, Trainers/Supporters/Stadiums and Energy you have in there. For instance, Heatmor, a deck that thrives off discarding Energy from ones deck for damage, has around fifty Energy and only a handful of Pokemon and Trainers in the deck. But normally, with a stage 2 deck, you are going to have around... Fifteen Pokemon, say, with about thirty-five Trainers, filling the rest with Energy? From my experience, that's how many seem to be in there. Of course, with Basic Pokemon decks, those numbers would be much different.
And yes, there is a 60 card mandatory count for a deck, meaning you can't have more, and you can't have less. There is an exact amount of cards for every deck, as stated in the rules of the game.