7, but I have owned 9.
2 copies of 1st edition hardcover (incorrect "Vollman"); one signed
1st edition hardcover (corrected "Vollmann")
Back Bay paperback, my 1st copy, heavily marked-up; reading copy
Abacus UK paperback edition
Uncorrected Advance Proof - signed
10th anniversary paperback
I thought I had a 2nd signed 1st edition hardcover, but I may have given it away when my wife gave me the one listed above (not remembering I had one already). I also had a book club paperback (with the hard cover art) that I cut up into smaller pieces for easier transport way back when. I don't think I have that anymore.
And I have resisted buying the Italian and German translations .. so far.