Depending on my deck. Blastoise/Gatr - 15 Water, SP decks - 6-10, I have quite a few different decks. but I go typically any where between 5 and 15, and 15 was my average in the old base set days as well. So not much has really changed I guess.
I tend to use a bit too much energy because I fear drought, because of bad prizes. I currently run like 10 in Crobatech, in Nidoqueen I ran 12 or so, in Speedrill 8. LuxChomp would also be around 10: 4 Basic, 2 Call, 4 DCE.
Most deck use under 14 energy cards nless energy plays some vital role in the deck to say boast attack damage or draw.
I usally play no more then 9 energy depending on the deck when I use poliwrath I dont need as many energy as I h ave DCE but say in My Torterra I play 12 energy since it gets its power from energy cards being attached.
Post-Rotation wise once we lose Roseanne's in favor of Interviewer's I'd say the standard amount will be around 15-16 maybe more depending on deck space and what we lose from DP-On. Some might include DCE since it will still be legal.
Currently I run about 13 energies in my BlastiGatr deck and 12 in my CharPhlosion deck.
You start out with a hand of seven cards, and there is a large chance that there will be at least one energy in your hand. If not, you might have some Rosy's or stuff like that.