That's right. It's preferable to go for 10-15 pokémon, 30-40 trainer and supporters, and 10-15 energies. As long as you can guarantee a basic pokémon in your starting hand, many trainer cards let you get the other basics and evolutions that you need. 15/30/15 is a more casual friendly mix, the tighter ones are used by people with optimized strategies.
It's also good to have 4 copies of whatever basic pokémon you want to focus on, as well as anything else that's necessary for your main strategy.
I play it casually but I held myself against the utterly savage folks on the TCG app's ranked mode to have some idea of the basics.