What's the difference? vs What are the differences?

[ref1]

"What's the difference between A and B?"
"What are the differences between A and B?"

Which one do you ask when you don't know how many differences there are?
I heard my teacher said "Who has not finished...?". Does this mean the singular form is used more common? I usually don't hear "Who have not finished...?"

 

[5jj]

Which one do you ask when you don't know how many differences there are?

The singular is probably more likely.
I heard my teacher said "Who has not finished...?".

'Who' as subject is followed by a singular verb.