What is a good Pokemon deck ratio?
I play regularly with my kids.  The energy card count depends heavily on the pokemon you use.  I have gone as low a 8 in a deck where all the pokemon had effective attacks at 1 energy and a retreat cost of 0 or 1.  I have gone as high as 28 or 30 when the deck was based on pokemon that discarded energy to attack and/or took 3 or 4 energy to have an effective attack.

When building a new deck we usually start at 16-18 energy adjusting up or down by a few as outlined above.   Then we tweak after some playing games.  My kids, who have more time than I do, will often build a deck then "goldfish" it.  ie play out a game solo to see if the needed energy arrive as fast as they can be played.