The Evocation Wizard's Sculpt Spell ability allows the wizard to protect some creatures from their own evocation spells:

When you cast an Evocation spell that affects other creatures that you can see, you can choose a number of them equal to 1 + the spell’s level. The chosen creatures automatically succeed on their Saving Throws against the spell, and they take no damage if they would normally take half damage on a successful save.

Does the number of chosen creatures need to be exactly equal to 1 + the spell's level, or can it be lower?

For example, if an evocation wizard casts Fireball, can they choose 1, 2, or 3 creatures to be protected from the spell, or do they need to select either zero or exactly 4 creatures?