When attacking, can a player move one unit to the battlefield, realize they need more power, and then add another unit before passing, or must all attacking units be declared simultaneously?

Ruling: All units must be moved together when attacking. Moving units does not create a chain unless there's a trigger from that move, and once you move a unit into an enemy-controlled battlefield, you've started Combat.

Nuances:

• You can immediately undo and redo the move with additional units if you haven't gained any new information in between and a judge allows it (e.g., "I move this unit... ah no, I move these two")
• You cannot undo the move if you only realized the mistake after your opponent revealed new information (such as pointing out an Ahri trigger)
• At lower organized play levels, the opponent may allow takebacks even in situations where they wouldn't normally be permitted