"Am cele mai puține fructe."

Translation: I have the fewest fruits.

[LovLviv]

What would be, I have the smallest fruits?

[potestasity]

Am cele mai mici fructe.

[WarsawWill]

As far as I know, in normal use, it is not that the singular and plural are the same, but that "fruit" is uncountable and doesn't have a plural form (except in expressions like "citrus fruits"). We don't say "He had two fruit for breakfast", but "He had two pieces of fruit for breakfast".

So personally, I'd just say "I have the least fruit". Which was accepted.

In Google:
"have the least fruit" - 20
"have the least amount of fruit" - 10
"have the fewest fruit" - 5 + this page
"have the fewest fruits" - only this page