The 20/20/20 deck building principle has rarely if ever, worked for competitive Pokémon play. Meaning I
think it might have for a short time in one of the earlier Modified Formats.
For competitive play,
low quantity Trainer decks are still going to have 25-30 total Trainer cards in them, with Trainer
heavy decks clocking it at around 40 to 45. Something in the middle is going to be between 30 and 40 Trainers. Expect 10-15 Items, 0-4 Stadiums, and 10-15 Supporters.
Currently, decks are actually running "low quantity" amounts in Standard Format play because of what is currently good.
Garbodor (
SM: Guardians Rising 51/145) and its "Trashalanche" attack do damage based on how many Item cards are in the other player's discard pile, so decks are trying to cut back on Item cards. Normally, Stage 2 Pokémon aren't very strong contenders, but right now
Gardevoir-EX,
Greninja BREAK (which actually Evolves
from a Stage 2) are among the best decks; they also need a good deal of Energy (12 for
Gardevoir-GX, 10 for
Greninja BREAK), that means cutting back on Trainer card.
You can look up recent Regional Championship results
here. That's a link to the official page, should it matter. You'll see the Top 8 for each age group, complete with decklists. Some of those events use the Expanded Format, instead of Standard; that is where you'll see decks with even higher Trainer counts.