Sai caught 10 fishes without an I,

9 were deprived of tail

6 were headless

And half of 8

He weighed on a trusted scale

Can you find out if he asks you?

How many fishes were there in his basket?

Zero

He catches 10 fishes without an I or 1 according to Roman numeral. Taking off 1 from 10 makes it 0. Cutting the tail of 9 makes it zero. Removing the head of 6 makes it zero. Slicing 8 in half makes it zero as well. Thus in total, he caught zero fishes.