"numara" is mostly used to describe things like telephone numbers, numbers in line, etc.
"sayı" is what is used in math and is like a digit.
Couldn't it be "bu sayı hangi", or "hangi bu sayı"?
I translated it as "which is this number", but wasn't accepted (correct: which ONE is this number).
I translated it the same way as you. After thinking about it, I believe you have to leave together 'hangi sayı' for 'which number' and the 'bu' you may put in front of or behind it.
So it could be (not for sure):
Bu hangi sayı ? or
Hangi sayı bu ?