The ability is poorly written, and the intent is clear.
I admit, in a strict RAW sense, you can either choose 1+level or 0. Sometimes strictly RAW readings give less than desirable results - this is one of those cases.
Let’s consider fireball at 3rd level. My friend is engaged in melee combat with two enemies, and I’d like to blast them with fireball without torching my friend.
Here is what a strict reading of the ability means:
I have the skill to shape my fireball around four allies, but not one, two, or three. Oh, and if I upcast fireball to 4th level, I forget how to shape it around four and can only do five.
This is silliness. Just let it be a number of creatures up to 1 + the spell’s level.