Obviously it depends on the deck, but in general you want a large number of item cards and less pokemon and energy.
Item/trainer cards provide a lot of ways to draw/recycle and support your pokemon/energy and you’ll likely want to see a bigger ratio of them in your opening hand. Especially since you’re not going to go through too many more pokemon than you have prize cards and you can only have 1 active pokemon at a time with 6 total in play.
So the exact ratio depends on the deck and the pokemon you are trying to support, but essentially you shouldn’t be running more the 15 pokemon and 15 energy on a high end. 12 pokemon is pretty standard. Most of the rest of your deck should be search and draw cards which there should be plenty of.