What is the correct form of question:
a) “How many sets do you have left?”
b) “How many sets you have left?”
c) “How many sets you got left?”
And also, what is the rule that I need to use to know when I should put "do" before "you" (I mean "do you have left"/"you have left")?
As Lawrence mentions in his comment, "How many are left?" "How many do you have left?" and, "How many have you got left?" are common and broadly interchangeable. The first is more a question of existence while the second two are about possession, but in most contexts these mean the same thing, since you would be asking about the ones the listener currently has.
Do you have the sandwiches? How many are left?
How many sandwiches do you have left?
How many sandwiches have you got left?
"How many have you left?" is slightly more formal but otherwise means the same thing. You may find it more commonly used in certain dialects (possibly BrE).
"How many you got left?" is dialect, and very informal. Again, it may be more common and acceptable in the vernacular of certain regions (possibly AmE more than BrE).
