I have experience with MTG but only a passing knowledge of Pokemon. My understanding is that it would depend entirely on what your deck does. Are you using pokemon that are expensive to use? Do you have any means of getting energy besides just drawing it? Do you have ways to draw more cards? All these things will come into play to determine the ratio you need. With experience you can guesstimate these things, but to be sure, the only way is to play the deck a bunch and adjust depending on what you feel you need to add.

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.

Disclaimer: I don't play this game.

I'd recommend dabbling in the tcg live app on mobile/pc, i believe you can enter your deck codes if you still have them & practice with the current legal standard