I am aware of approximately 170 surviving pieces of published Oz artwork by John R. Neill. There are also a number of unpublished pieces, sketches, etc. I know that's not a complete number, but even if it were doubled - which seems unlikely - it would be a far cry from the amount of work Neill did for the series. He illustrated 35 Oz books, plus the Little Wizard series. Early advertising for the Oz books states that each book has over 100 illustrations - some have quite a few more - so out of over 3500 pieces of art, the number of surviving pieces is really quite small.
One of the largest archives of remaining Oz artwork was sold through Books of Wonder in 1984. Their Art of Oz catalog contains pieces that had remained in the publisher's files until that time. I still drool over some of the pieces in that catalog!
Illustration art was not considered of much value, in the not so distant past. I've heard stories, whether true or not, of Oz illustrator/author Dick Martin pulling artwork out of dumpsters after Reilly & Lee was bought by Henry Regnery Co. While it's difficult to believe that anyone would simply dispose of such wonderful things, it was common practice. Fortunately not everything was destroyed, so there are still original pieces to be enjoyed today.