On the turn after using Dragonair's "Dragon's Wish" attack, when I attach any number of Energy cards from my hand do I have to attach them all at once, or can I attach them whenever I want during my turn?
You do not have to attach them all at once. You can attach them from your hand whenever you like during the duration of your turn following Dragon's Wish.
Source: Sun & Moon FAQ (2017-02-02), TPCi Rules Team (2017-02-02)

Relevant cards

Note: All applicable cards may not be listed below.
SM1_95