When using the evocation wizard’s Sculpt Spells, can you protect fewer creatures than the maximum you are allowed?

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?