As far as I can tell, j. d., the one volume contains the lot of it. I don't see any indication of it being one of a set. Anyway, the introduction is titled "Thirty-Two Soothsayers", and there appear to be at least that many stories in the book. And, yes, it is fairly long...514 pages.
I had hoped to pick up all the volumes of Again, Dangerous Visions, but the store where I got it had three or four copies of volume two and no copies that I could find of volume one. Oh, well. Part is better than none. I just wish I still had the hardbacks of both books, but they got lost somewhere along the way.
By the way, the list of books might have been much longer if I had been able to spend more time in Acres of Books. Having finally visited there, I fully believe their claim to have around a million volumes in stock.
It was a little like Christmas in April, Allegra. The amazing thing to me is that I spent less than a third of the money I had allowed myself for book purchases. And, goodness knows, I spent more on the four books I bought new than I did on the fifteen I bought used.