How to Rend Fiends and Immolate People: A Guide to Sorcery

[SharkForce]

Careful Spell:
When you cast a spell that forces other creatures to make
a saving throw, you can protect some of those creatures
from the spell's full force. To do 50, you spend 1 sorcery
point and choose a number of those creatures up to your
Charisma modifier (minimum of one creature). A chosen
creature automatically succeeds on its saving throw[singular]
against the spell.

It's not RAI, it's RAW. One saving throw and only when you cast the spell (e.g. the same turn).

If you're making a guide the assumption is that people are playing by the rules....

when you cast a spell, yes. but not when you cast a spell, they automatically pass their save. when you cast a spell, you choose creatures to protect.

then, when those creatures have to make a saving throw against the spell (which they only ever need to make one at a time), they automatically pass. no limitation to one saving throw. no limitation to on your turn. it isn't RAW, and if it's RAI then it's a steaming pile of turds, and as i said they can take their stupid RAI and shove it where the sun don't shine, because that's where it belongs.