Oracle Text

This spell costs 1 more to cast for each target beyond the first.

Fireball deals X damage divided evenly, rounded down, among any number of targets.

Card Rulings

11/17/2017 For example, if X is 5 and you choose three target creatures, Fireball has a total cost of (even though its mana cost is just and its converted mana cost is 6). If those creatures are all still legal targets as Fireball resolves, it deals 1 damage to each of them.
11/17/2017 You may cast Fireball with zero targets, regardless of the value chosen for X. If you do so, it will not be a targeted spell, and no damage will actually be dealt when it resolves.
11/17/2017 Fireball’s damage is divided as Fireball resolves, not as it’s cast, because there are no choices involved. The division involves only targets that are still legal as Fireball resolves.
11/17/2017 You can target more than X creatures. However, if the number of legal targets at the time Fireball resolves is greater than X, none of them will be dealt any damage.