Ah good one there, and i think it can be quite different for each book, for instance. Good omens , i know there was a proof copy with 4th adjusted text, so was there a 1st 2nd and 3rd, or a 5th for that matter, as ive never come across one in my travels i really cant answer that, after the 4th it will go to a proof/arc, now it depends on where you come from for it to be called what it is, USA is an ARC, UK is a proof, so in the case of good omens there were 2 different arc's produced in the USA and one proof in the UK so there's your answer.
Oh for the normal Discworld books, i know there are some 4th adjusted text proofs out there, which then lead on to the actual proof/arc, so the best way to look at it that normally there is only one proof produced per copy before the book is published.
I hope this helps
Oh i should add the 4th adjusted text copies normally never get out to the public, they normally stay with the writer or the publisher, and the fact that i have one dosnt come into it