I don't have any rules about how many drafts I'll do for a book. Each books takes as many drafts as it takes to make it the best it can possibly be. I did quite a lot of work during the second draft of the book, but I still wasn't happy with certain parts so moved onto the third draft. There were a few things I needed to work on, like the pacing in some places and the plot gets a little heavy at times, so I wanted to lighten it with a bit more humour, and I had little niggly bits to sort out, like the name-switching of a very minor character who was called both Mike and Mick (which, to be fair, are practically the same). But the main focus was on the ending. It was a bit wishy-washy for my liking, so a lot of the scenes were heavily tweaked or completely cut and re-written. The ending is a lot stronger now, with the main character moving it along rather than being a bit of a bystander.
I'm now happy with how the book reads at this stage, so I'll do another read-through (and no doubt tweak here and there), and then it'll be sent off to an editor.