Flex Caster feat — how does Upcast work?

English is not my native language, so I’m not sure if I’m interpreting the wording of the Upcast feature correctly.

The feature says:

Upcast. When you cast a spell, such as Magic Missile, that can be cast with a higher-level spell slot for more powerful effects, you can expend any additional spell slot to increase the spell’s effective level by 1, to a maximum of level 9.

My question is: can I spend multiple additional spell slots on the same casting to keep increasing the spell’s effective level, potentially all the way up to 9th level? Or can I only spend one additional spell slot, increasing the spell’s effective level by exactly 1?

For example, if I cast Magic Missile using a 1st-level spell slot, would I be allowed to spend several other spell slots to raise its effective level from 1st to 2nd, then 3rd, then 4th, and so on, up to 9th level?

Or does the feature simply mean that I may spend one additional spell slot to make the spell count as one level higher, so a 1st-level Magic Missile could only become effectively 2nd level through this feature?

The part that confuses me is the phrase “any additional spell slot.”

From what I understand about English grammar, “spell slot” is a singular countable noun, so “any additional spell slot” would normally mean something like “any one additional spell slot” or “whichever additional spell slot you choose.” In other words, “any” seems to indicate that I can choose which slot to expend, rather than indicating any number of slots.

If the intent were to allow multiple slots to be spent, I would have expected wording such as:

“You can expend any number of additional spell slots...”

or:

“For each additional spell slot you expend, increase the spell’s effective level by 1.”

or:

“You can expend one or more additional spell slots...”

Because none of those expressions appear in the feature, my current interpretation is that you spend one additional spell slot and increase the spell’s effective level by 1, with the “maximum of level 9” simply preventing the effective spell level from ever going above 9.

However, since English is not my first language, I’m not sure whether I’m reading “any additional spell slot” too literally. Does the RAW wording allow spending multiple slots on a single casting, or only one?

In my opinion the RAW and RAI is to allow exactly one additional spell slot.  

So for example as a 5th level Wizard that has access to 3rd level spell slots you could normally upcast Magic Missile to be cast as a 3rd level spell by using your 3rd level spell slot.  With this feat you could now cast the spell with your 3rd level spell slot and also use a 1st level spell slot at the same time so that the spell could be cast as a 4th level spell.  But you cannot cast it as a 9th level spell at that time.