My honest answer, though given with tongue firmly in cheek, is that the number is always one and legion.
As I write, I edit, often at the level of the individual word. I may stop typing a word to replace it with another one then replace that with another and so on several times before I move on. When I have finished the sentence I may go back and polish it ten times before moving on to the next. When I have finished the paragraph I may go back and polish the joined together sentences a dozen times. When first finishing a chapter I may go back over it twenty times before moving on to the next. Yet, contradictorily, I may have a day of flow, usually after weeks of laboring over detail, where two thousand words burst forth perfectly at the first needing nothing more than the proper placement within the greater whole. These different levels of creation and editing are ongoing, the text is a living growing thing until that final day when I read through the whole thing and say “There! This is the moment I capture, this is the book complete.” If you count every change of word or punctutation then the number of drafts is in the thousands. If you count as a draft only that point when I say I think it is done, then the number of drafts is one, one draft labored over word by word.