Oracle Text

Choose a number. Destroy all artifacts and creatures with converted mana cost equal to that number. Then target player reveals their hand and discards all nonland cards with converted mana cost equal to the number.

Card Rulings

6/8/2016 You can choose 0. Artifact lands and most tokens have converted mana costs of 0, for example.
6/8/2016 The target player is chosen as you announce Void, but the number isn’t chosen until it’s resolving. No player may choose to take actions between you choosing a number and the rest of Void’s effects.