I just got 5/6 on a stage.

It's not the number of supers I had left by the end.
It's not the number of supers I used.
It's not the number of supers I collected.

Actually, I do think it seem to be related to the number of supers you did, or super... bars? like, each of the cards you used?. I do agree it is not a 'how much I charged'

I'm pretty sure that I used two supers during that.
Could it be... the number of super symbols consumed during a single move? And one has to buy some item to have 6 instead of 5 cards before even being able to get 6/6?

So apparently, it's the number of supers used. Guess a special counts as 5 but one could also use 6 EX moves.