RAW: no

As you say, the ability is not actually ambiguous:

you can choose a number of them equal to 1 + the spell’s level.

Contrast this to the sorcerer's Careful Spell Metamagic:

spend 1 sorcery point and choose a number of those creatures up to your Charisma modifier (minimum of one creature).

If Sculpt Spell was supposed to give you the ability to target fewer creatures, it would presumably have equal replaced by up to give:

choose a number of them up to 1 + the spell’s level.

One possible counter argument is that you could choose the same creature multiple times, thereby de facto granting the ability to target fewer creatures. However, I would argue that this is intentional butchering the language of the ability, and thus not really RAW.

Another possible objection is that the ability includes the word can to indicate that it is not required to be used. However, this only refers to the ability as a whole. That is, you can either protect 0 creatures or 1 + the spell's level.

RAI: likely yes

Requiring that you protect the maximum number of creatures seems like a silly requirement. It would sometimes make it beneficial for a friendly character to move in range of your fireball, which is the kind of design that does not fit with the rest of the game. Furthermore, this is the kind of error that can go unnoticed while writing and testing a book.

There has been no sage advice on this, nor has it been featured in errata, but I think that if it were the meaning would be changed to up to 1 + the spell’s level"up to 1 + the spell’s level".

Every DM I know has ruled it as up to, but technically this does not conform to RAW.