Wikipedia puts him at 40 novels, enough short stories for maybe 40 short to mid-sized collections (382 stories), 280 non-fic books, and 146 as editor or annotator, which surely don't count. 504 books included edited/annotated. If one were only using fiction and excluding duplicates (I'm sure there were collections of short stories of his which were mostly or entirely repeats from earlier collections) then that's maybe 80 volumes of unique material (I'm sure there were tons of dupes in various story collections), tops. They'd easily fit in 40-50 fairly ordinary vols. I bet if you were willing to put multiple "books" in one (a lot of his juvenilia are really short) and probably 10-15 LOA-style volumes.

Not that that's a small amount of fiction to have written, but even including the non-fic it's nowhere near 900.

Yes, seems my off-the-cuff memory's is counting an individual short story as a book & even then doubled the value

http://www.asimovonline.com/oldsite/asimov_catalogue.html