Flygon's first move is called Reactive Blast and only takes 2 energy cards, 1 colorless, 1 electric and does 40+ damage depending on if you want to discard any react energy cards from Flygon while you perform the move, so in essence, Reactive Blast can do 100 damage for 2 energy cards, if both your energy cards attached to Flygon are react energy cards
If you want to talk numbers, Id say 8-10 pieces of mana ramp is a good number to aim for, but thats very arbitrary
Lightning Bolt | Illustration by Christopher Moeller These are instants and sorceries that cost as little mana as possible
The second mode drains your opponents, and the third gives you card advantage for each opponent with one or fewer cards in hand, which is relevant if your deck is built around making your opponents discard their hand
This leaves a wide range of possibilities open