As many as it takes. I’m working on the third draft of The Warden, but the truth is that I’ve been through that draft more than once, so I’m not really sure how to number drafts, assuming that I wanted to be accurate about it. What seems to happen is that I go through a draft as many times as it takes for me to be provisionally satisfied with it. Then, if there’s time, I let it rest a while before creating the next numbered draft. I’ll probably settle for four for this novel, mainly because I still haven’t written the last chapters, and those will need the most going over before I’m satisfied with them.
Right now, I’m keeping an eye out for the picky stuff like punctuation errors and duplicate or dropped words, but mostly, I’m tweaking sentences to run more smoothly, and picking up on the tiny things that most readers won’t even notice. Sentences that don’t refer as clearly as they should to something in the past, or small continuity errors. And then are the errors that slip in, unnoticed, when you’re correcting the original errors.
Whatever the actual number of the last draft, the very last thing I do with it is line editing, which is the hardest and nastiest part of the whole job, as far as I’m concerned. This is the one that requires a change of font and font size, in order to force the eyes to see things anew. And patience and concentration, above all.