The exact reading of the ability is:
Beginning at 2nd Level, you can create pockets of relative safety
within the Effects of your 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.
I think the natural way to read this, both as RAW and as RAI is that the first portion of the ability specifies the what and the second portion specifies the how and the limitations (how many).
Remember that all of these Wizard School abilities are optional if they say "can" initially.
you can create pockets of relative safety within the Effects of your
This part covers the question of "do you select 0 creatures or a number of creatures." Well, choosing not to use the ability at all would be selecting 0, otherwise why even use the ability?
So when the second portion uses the word "can" it is specifying the option to use the ability's maximum number or not.
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
This portion specifies that you have the option to select any number of creatures (ally OR enemy) which could be limited by what you literally have the ability to see, which may limit the ability to protect the maximum number of creatures possible anyways as there are often vision obstructing objects in the battlefield. Keep this in mind.
Compare this reading to another ability from the same school which seems to be leaving you no choice in the matter:
Beginning when you select this school at 2nd level, the gold and time
you must spend to copy a Evocation spell into your spellbook is
This ability does not use the word can, or leave any real way for you to interpret this as an optional ability. It simply is halved, the end. The RAW states that it just DOES cost less time and gold for you to copy Evocation spells, and the RAI very clearly want to convey the sense that as a now seasoned(ish) Evocation wizard, you are better at time and money management when it comes to these spells specifically.
Now compare this to a few abilities that read fairly similarly to Spell Sculpt.
All of these abilities have the specific reading "can x a number of x equal to":
...you can use your Reaction when you fall to reduce any
Falling damage you take by an amount equal to five times your monk
...You can use this feature a number of times equal to 1 + your
Charisma modifier. When you finish a Long Rest, you regain all
...Each creature can gain temporary hit points equal to your level +
your Charisma modifier. A creature can't gain temporary hit points
from this feat again until it has finished a short or long rest...
Something that all of these abilities have in common is that there could be instances where you don't need to use the maximum possible effect.
You may only need to negate some fall damage because you fell a short distance, or you may only need to detect the presence of a celestial, fiend or undead once a day, or a companion may have 12 temporary hit points and when you try to give them 10, they opt to keep their 12 and not take your 10.
Similarly, it stands to reason that your adventuring party may have only ever been 2 other people and you will never need to use Sculpt Spell on more than 2 or 3 people, regardless of the spell level you are using.
I think it's counter-intuitive to presume that the reading of "can x a number of x equal to" means that you MUST adhere to a strict definition of the maximum possible.
There ARE instances in PHB that use words like "equal to or less than" "up to" and "minimum of 1" but there are many that specify that you have no choice using words like "must" "is" or simply lack any language that would imply there is a choice.
Your hit point maximum increases by an amount equal to twice your
level when you gain this feat. Whenever you gain a level thereafter,
your hit point maximum increases by an additional 2 hit points.
This ability omits any language that would specify that it is either a requirement or a choice, but there is really only one way to interpret this.
If you're bothering to take this feat in the first place, why in the world would you treat it as an option?
Remember as well that the ability specifies creatures you can see which, if we read this ability to mean "all or nothing" then that means that Spell Sculpt is literally only useful if you have the exact number of party members as the spell you are trying to cast+1 and if you can see them all at the same time. Otherwise you would need to start targeting enemies as part of Spell Sculpt and at that point we have entered into pure silly territory.
In conclusion, Spell Sculpt very clearly adheres to natural English language rules. It neither omits nor includes any language that implies it is required that you use the ability to the maximum effect, and it specifies twice that there is the option to both use the ability at all AND use it to its maximum effect.